first commit

core/modules/layout_builder/src/Access/LayoutBuilderAccessCheck.php

@@ -0,0 +1,51 @@
+<?php
+
+namespace Drupal\layout_builder\Access;
+
+use Drupal\Core\Access\AccessResult;
+use Drupal\Core\Cache\RefinableCacheableDependencyInterface;
+use Drupal\Core\Routing\Access\AccessInterface;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\layout_builder\SectionStorageInterface;
+use Symfony\Component\Routing\Route;
+
+/**
+ * Provides an access check for the Layout Builder defaults.
+ *
+ * @ingroup layout_builder_access
+ *
+ * @internal
+ *   Tagged services are internal.
+ */
+class LayoutBuilderAccessCheck implements AccessInterface {
+
+  /**
+   * Checks routing access to the layout.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   * @param \Drupal\Core\Session\AccountInterface $account
+   *   The current user.
+   * @param \Symfony\Component\Routing\Route $route
+   *   The route to check against.
+   *
+   * @return \Drupal\Core\Access\AccessResultInterface
+   *   The access result.
+   */
+  public function access(SectionStorageInterface $section_storage, AccountInterface $account, Route $route) {
+    $operation = $route->getRequirement('_layout_builder_access');
+    $access = $section_storage->access($operation, $account, TRUE);
+
+    // Check for the global permission unless the section storage checks
+    // permissions itself.
+    if (!$section_storage->getPluginDefinition()->get('handles_permission_check')) {
+      $access = $access->andIf(AccessResult::allowedIfHasPermission($account, 'configure any layout'));
+    }
+
+    if ($access instanceof RefinableCacheableDependencyInterface) {
+      $access->addCacheableDependency($section_storage);
+    }
+    return $access;
+  }
+
+}

core/modules/layout_builder/src/Access/LayoutPreviewAccessAllowed.php

@@ -0,0 +1,28 @@
+<?php
+
+namespace Drupal\layout_builder\Access;
+
+use Drupal\Core\Access\AccessibleInterface;
+use Drupal\Core\Access\AccessResult;
+use Drupal\Core\Session\AccountInterface;
+
+/**
+ * Accessible class to allow access for inline blocks in the Layout Builder.
+ *
+ * @internal
+ *   Tagged services are internal.
+ */
+class LayoutPreviewAccessAllowed implements AccessibleInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function access($operation, ?AccountInterface $account = NULL, $return_as_object = FALSE) {
+    if ($operation === 'view') {
+      return $return_as_object ? AccessResult::allowed() : TRUE;
+    }
+    // The layout builder preview should only need 'view' access.
+    return $return_as_object ? AccessResult::forbidden() : FALSE;
+  }
+
+}

core/modules/layout_builder/src/Annotation/SectionStorage.php

@@ -0,0 +1,70 @@
+<?php
+
+namespace Drupal\layout_builder\Annotation;
+
+use Drupal\Component\Annotation\Plugin;
+use Drupal\layout_builder\SectionStorage\SectionStorageDefinition;
+
+/**
+ * Defines a Section Storage type annotation object.
+ *
+ * @see \Drupal\layout_builder\SectionStorage\SectionStorageManager
+ * @see plugin_api
+ *
+ * @Annotation
+ */
+class SectionStorage extends Plugin {
+
+  /**
+   * The plugin ID.
+   *
+   * @var string
+   */
+  public $id;
+
+  /**
+   * The plugin weight, optional (defaults to 0).
+   *
+   * When an entity with layout is rendered, section storage plugins are
+   * checked, in order of their weight, to determine which one should be used
+   * to render the layout.
+   *
+   * @var int
+   */
+  public $weight = 0;
+
+  /**
+   * Any required context definitions, optional.
+   *
+   * When an entity with layout is rendered, all section storage plugins which
+   * match a particular set of contexts are checked, in order of their weight,
+   * to determine which plugin should be used to render the layout.
+   *
+   * @var array
+   *
+   * @see \Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface::findByContext()
+   */
+  public $context_definitions = [];
+
+  /**
+   * Indicates that this section storage handles its own permission checking.
+   *
+   * If FALSE, the 'configure any layout' permission will be required during
+   * routing access. If TRUE, Layout Builder will not enforce any access
+   * restrictions for the storage, so the section storage's implementation of
+   * access() must perform the access checking itself. Defaults to FALSE.
+   *
+   * @var bool
+   *
+   * @see \Drupal\layout_builder\Access\LayoutBuilderAccessCheck
+   */
+  public $handles_permission_check = FALSE;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function get() {
+    return new SectionStorageDefinition($this->definition);
+  }
+
+}

core/modules/layout_builder/src/Attribute/SectionStorage.php

@@ -0,0 +1,65 @@
+<?php
+
+namespace Drupal\layout_builder\Attribute;
+
+use Drupal\Component\Plugin\Attribute\Plugin;
+use Drupal\layout_builder\SectionStorage\SectionStorageDefinition;
+
+/**
+ * Defines a SectionStorage attribute.
+ *
+ * Plugin Namespace: Plugin\SectionStorage
+ *
+ * @see \Drupal\layout_builder\SectionStorage\SectionStorageManager
+ * @see plugin_api
+ */
+#[\Attribute(\Attribute::TARGET_CLASS)]
+class SectionStorage extends Plugin {
+
+  /**
+   * Constructs a SectionStorage attribute.
+   *
+   * @param string $id
+   *   The plugin ID.
+   * @param int $weight
+   *   (optional) The plugin weight.
+   *   When an entity with layout is rendered, section storage plugins are
+   *   checked, in order of their weight, to determine which one should be used
+   *   to render the layout.
+   * @param \Drupal\Component\Plugin\Context\ContextDefinitionInterface[] $context_definitions
+   *   (optional) Any required context definitions.
+   *   When an entity with layout is rendered, all section storage plugins which
+   *   match a particular set of contexts are checked, in order of their weight,
+   *   to determine which plugin should be used to render the layout.
+   *   @see \Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface::findByContext()
+   * @param bool $handles_permission_check
+   *   (optional) Indicates that this section storage handles its own
+   *   permission checking. If FALSE, the 'configure any layout' permission
+   *   will be required during routing access. If TRUE, Layout Builder will
+   *   not enforce any access restrictions for the storage, so the section
+   *   storage's implementation of access() must perform the access checking itself.
+   * @param string|null $deriver
+   *   (optional) The deriver class.
+   */
+  public function __construct(
+    public readonly string $id,
+    public readonly int $weight = 0,
+    public readonly array $context_definitions = [],
+    public readonly bool $handles_permission_check = FALSE,
+    public readonly ?string $deriver = NULL,
+  ) {}
+
+  /**
+   * {@inheritdoc}
+   */
+  public function get(): SectionStorageDefinition {
+    return new SectionStorageDefinition([
+      'id' => $this->id,
+      'class' => $this->class,
+      'weight' => $this->weight,
+      'context_definitions' => $this->context_definitions,
+      'handles_permission_check' => $this->handles_permission_check,
+    ]);
+  }
+
+}

core/modules/layout_builder/src/Cache/ExtraFieldBlockCacheTagInvalidator.php

@@ -0,0 +1,37 @@
+<?php
+
+namespace Drupal\layout_builder\Cache;
+
+use Drupal\Component\Plugin\Discovery\CachedDiscoveryInterface;
+use Drupal\Core\Block\BlockManagerInterface;
+use Drupal\Core\Cache\CacheTagsInvalidatorInterface;
+
+/**
+ * Provides a cache tag invalidator that clears the block cache.
+ *
+ * @internal
+ *   Tagged services are internal.
+ */
+class ExtraFieldBlockCacheTagInvalidator implements CacheTagsInvalidatorInterface {
+
+  /**
+   * Constructs a new ExtraFieldBlockCacheTagInvalidator.
+   *
+   * @param \Drupal\Core\Block\BlockManagerInterface $blockManager
+   *   The block manager.
+   */
+  public function __construct(protected BlockManagerInterface $blockManager) {
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function invalidateTags(array $tags) {
+    if (in_array('entity_field_info', $tags, TRUE)) {
+      if ($this->blockManager instanceof CachedDiscoveryInterface) {
+        $this->blockManager->clearCachedDefinitions();
+      }
+    }
+  }
+
+}

core/modules/layout_builder/src/Cache/LayoutBuilderIsActiveCacheContext.php

@@ -0,0 +1,98 @@
+<?php
+
+namespace Drupal\layout_builder\Cache;
+
+use Drupal\Core\Cache\CacheableMetadata;
+use Drupal\Core\Cache\Context\CalculatedCacheContextInterface;
+use Drupal\Core\Entity\Entity\EntityViewDisplay;
+use Drupal\Core\Entity\FieldableEntityInterface;
+use Drupal\Core\Routing\RouteMatchInterface;
+use Drupal\layout_builder\Entity\LayoutEntityDisplayInterface;
+
+/**
+ * Determines whether Layout Builder is active for a given entity type or not.
+ *
+ * Cache context ID: 'layout_builder_is_active:%entity_type_id', e.g.
+ * 'layout_builder_is_active:node' (to vary by whether custom layout overrides
+ * are allowed for the Node entity specified by the route parameter).
+ *
+ * @internal
+ *   Tagged services are internal.
+ */
+class LayoutBuilderIsActiveCacheContext implements CalculatedCacheContextInterface {
+
+  /**
+   * The current route match.
+   *
+   * @var \Drupal\Core\Routing\RouteMatchInterface
+   */
+  protected $routeMatch;
+
+  /**
+   * LayoutBuilderCacheContext constructor.
+   *
+   * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
+   *   The current route match.
+   */
+  public function __construct(RouteMatchInterface $route_match) {
+    $this->routeMatch = $route_match;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getLabel() {
+    return t('Layout Builder');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getContext($entity_type_id = NULL) {
+    if (!$entity_type_id) {
+      throw new \LogicException('Missing entity type ID');
+    }
+
+    $display = $this->getDisplay($entity_type_id);
+    return ($display && $display->isOverridable()) ? '1' : '0';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getCacheableMetadata($entity_type_id = NULL) {
+    if (!$entity_type_id) {
+      throw new \LogicException('Missing entity type ID');
+    }
+
+    $cacheable_metadata = new CacheableMetadata();
+    if ($display = $this->getDisplay($entity_type_id)) {
+      $cacheable_metadata->addCacheableDependency($display);
+    }
+    return $cacheable_metadata;
+  }
+
+  /**
+   * Returns the entity view display for a given entity type and view mode.
+   *
+   * @param string $entity_type_id
+   *   The entity type ID.
+   *
+   * @return \Drupal\layout_builder\Entity\LayoutEntityDisplayInterface|null
+   *   The entity view display, if it exists.
+   */
+  protected function getDisplay($entity_type_id) {
+    if ($entity = $this->routeMatch->getParameter($entity_type_id)) {
+      if ($entity instanceof FieldableEntityInterface) {
+        // @todo Expand to work for all view modes in
+        //   https://www.drupal.org/node/2907413.
+        $view_mode = 'full';
+        $display = EntityViewDisplay::collectRenderDisplay($entity, $view_mode);
+        if ($display instanceof LayoutEntityDisplayInterface) {
+          return $display;
+        }
+      }
+    }
+  }
+
+}

core/modules/layout_builder/src/Cache/LayoutBuilderUiCacheContext.php

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+<?php
+
+namespace Drupal\layout_builder\Cache;
+
+use Drupal\Core\Cache\Context\RouteNameCacheContext;
+
+/**
+ * Determines if an entity is being viewed in the Layout Builder UI.
+ *
+ * Cache context ID: 'route.name.is_layout_builder_ui'.
+ *
+ * @internal
+ *   Tagged services are internal.
+ */
+class LayoutBuilderUiCacheContext extends RouteNameCacheContext {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getLabel() {
+    return t('Layout Builder user interface');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getContext() {
+    $route_name = $this->routeMatch->getRouteName();
+    if ($route_name && str_starts_with($route_name, 'layout_builder.')) {
+      return 'is_layout_builder_ui.0';
+    }
+    return 'is_layout_builder_ui.1';
+  }
+
+}

core/modules/layout_builder/src/Context/LayoutBuilderContextTrait.php

@@ -0,0 +1,55 @@
+<?php
+
+namespace Drupal\layout_builder\Context;
+
+use Drupal\Core\Plugin\Context\ContextInterface;
+use Drupal\layout_builder\SectionStorageInterface;
+
+/**
+ * Provides a wrapper around getting contexts from a section storage object.
+ */
+trait LayoutBuilderContextTrait {
+
+  /**
+   * The context repository.
+   *
+   * @var \Drupal\Core\Plugin\Context\ContextRepositoryInterface
+   */
+  protected $contextRepository;
+
+  /**
+   * Gets the context repository service.
+   *
+   * @return \Drupal\Core\Plugin\Context\ContextRepositoryInterface
+   *   The context repository service.
+   */
+  protected function contextRepository() {
+    if (!$this->contextRepository) {
+      $this->contextRepository = \Drupal::service('context.repository');
+    }
+    return $this->contextRepository;
+  }
+
+  /**
+   * Returns all populated contexts, both global and section-storage-specific.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   *
+   * @return \Drupal\Core\Plugin\Context\ContextInterface[]
+   *   The array of context objects.
+   */
+  protected function getPopulatedContexts(SectionStorageInterface $section_storage): array {
+    // Get all known globally available contexts IDs.
+    $available_context_ids = array_keys($this->contextRepository()->getAvailableContexts());
+    // Filter to those that are populated.
+    $contexts = array_filter($this->contextRepository()->getRuntimeContexts($available_context_ids), function (ContextInterface $context) {
+      return $context->hasContextValue();
+    });
+
+    // Add in the per-section_storage contexts.
+    $contexts += $section_storage->getContextsDuringPreview();
+    return $contexts;
+  }
+
+}

core/modules/layout_builder/src/Controller/AddSectionController.php

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+<?php
+
+namespace Drupal\layout_builder\Controller;
+
+use Drupal\Core\Ajax\AjaxHelperTrait;
+use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
+use Drupal\layout_builder\LayoutTempstoreRepositoryInterface;
+use Drupal\layout_builder\Section;
+use Drupal\layout_builder\SectionStorageInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+use Symfony\Component\HttpFoundation\RedirectResponse;
+
+/**
+ * Defines a controller to add a new section.
+ *
+ * @internal
+ *   Controller classes are internal.
+ */
+class AddSectionController implements ContainerInjectionInterface {
+
+  use AjaxHelperTrait;
+  use LayoutRebuildTrait;
+
+  /**
+   * The layout tempstore repository.
+   *
+   * @var \Drupal\layout_builder\LayoutTempstoreRepositoryInterface
+   */
+  protected $layoutTempstoreRepository;
+
+  /**
+   * AddSectionController constructor.
+   *
+   * @param \Drupal\layout_builder\LayoutTempstoreRepositoryInterface $layout_tempstore_repository
+   *   The layout tempstore repository.
+   */
+  public function __construct(LayoutTempstoreRepositoryInterface $layout_tempstore_repository) {
+    $this->layoutTempstoreRepository = $layout_tempstore_repository;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('layout_builder.tempstore_repository')
+    );
+  }
+
+  /**
+   * Adds the new section.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   * @param int $delta
+   *   The delta of the section to splice.
+   * @param string $plugin_id
+   *   The plugin ID of the layout to add.
+   *
+   * @return \Symfony\Component\HttpFoundation\Response
+   *   The controller response.
+   */
+  public function build(SectionStorageInterface $section_storage, int $delta, $plugin_id) {
+    $section_storage->insertSection($delta, new Section($plugin_id));
+
+    $this->layoutTempstoreRepository->set($section_storage);
+
+    if ($this->isAjax()) {
+      return $this->rebuildAndClose($section_storage);
+    }
+    else {
+      $url = $section_storage->getLayoutBuilderUrl();
+      return new RedirectResponse($url->setAbsolute()->toString());
+    }
+  }
+
+}

core/modules/layout_builder/src/Controller/ChooseBlockController.php

@@ -0,0 +1,267 @@
+<?php
+
+namespace Drupal\layout_builder\Controller;
+
+use Drupal\Core\Ajax\AjaxHelperTrait;
+use Drupal\Core\Block\BlockManagerInterface;
+use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\Core\Url;
+use Drupal\layout_builder\Context\LayoutBuilderContextTrait;
+use Drupal\layout_builder\LayoutBuilderHighlightTrait;
+use Drupal\layout_builder\SectionStorageInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Defines a controller to choose a new block.
+ *
+ * @internal
+ *   Controller classes are internal.
+ */
+class ChooseBlockController implements ContainerInjectionInterface {
+
+  use AjaxHelperTrait;
+  use LayoutBuilderContextTrait;
+  use LayoutBuilderHighlightTrait;
+  use StringTranslationTrait;
+
+  /**
+   * The block manager.
+   *
+   * @var \Drupal\Core\Block\BlockManagerInterface
+   */
+  protected $blockManager;
+
+  /**
+   * The entity type manager.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
+   */
+  protected $entityTypeManager;
+
+  /**
+   * The current user.
+   *
+   * @var \Drupal\Core\Session\AccountInterface
+   */
+  protected $currentUser;
+
+  /**
+   * ChooseBlockController constructor.
+   *
+   * @param \Drupal\Core\Block\BlockManagerInterface $block_manager
+   *   The block manager.
+   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
+   *   The entity type manager.
+   * @param \Drupal\Core\Session\AccountInterface $current_user
+   *   The current user.
+   */
+  public function __construct(BlockManagerInterface $block_manager, EntityTypeManagerInterface $entity_type_manager, AccountInterface $current_user) {
+    $this->blockManager = $block_manager;
+    $this->entityTypeManager = $entity_type_manager;
+    $this->currentUser = $current_user;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('plugin.manager.block'),
+      $container->get('entity_type.manager'),
+      $container->get('current_user')
+    );
+  }
+
+  /**
+   * Provides the UI for choosing a new block.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   * @param int $delta
+   *   The delta of the section to splice.
+   * @param string $region
+   *   The region the block is going in.
+   *
+   * @return array
+   *   A render array.
+   */
+  public function build(SectionStorageInterface $section_storage, int $delta, $region) {
+    if ($this->entityTypeManager->hasDefinition('block_content_type') && $types = $this->entityTypeManager->getStorage('block_content_type')->loadMultiple()) {
+      if (count($types) === 1) {
+        $type = reset($types);
+        $plugin_id = 'inline_block:' . $type->id();
+        if ($this->blockManager->hasDefinition($plugin_id)) {
+          $url = Url::fromRoute('layout_builder.add_block', [
+            'section_storage_type' => $section_storage->getStorageType(),
+            'section_storage' => $section_storage->getStorageId(),
+            'delta' => $delta,
+            'region' => $region,
+            'plugin_id' => $plugin_id,
+          ]);
+        }
+      }
+      else {
+        $url = Url::fromRoute('layout_builder.choose_inline_block', [
+          'section_storage_type' => $section_storage->getStorageType(),
+          'section_storage' => $section_storage->getStorageId(),
+          'delta' => $delta,
+          'region' => $region,
+        ]);
+      }
+      if (isset($url)) {
+        $build['add_block'] = [
+          '#type' => 'link',
+          '#url' => $url,
+          '#title' => $this->t('Create @entity_type', [
+            '@entity_type' => $this->entityTypeManager->getDefinition('block_content')->getSingularLabel(),
+          ]),
+          '#attributes' => $this->getAjaxAttributes(),
+          '#access' => $this->currentUser->hasPermission('create and edit custom blocks'),
+        ];
+        $build['add_block']['#attributes']['class'][] = 'inline-block-create-button';
+      }
+    }
+
+    $build['filter'] = [
+      '#type' => 'search',
+      '#title' => $this->t('Filter by block name'),
+      '#title_display' => 'invisible',
+      '#size' => 30,
+      '#placeholder' => $this->t('Filter by block name'),
+      '#attributes' => [
+        'class' => ['js-layout-builder-filter'],
+        'title' => $this->t('Enter a part of the block name to filter by.'),
+      ],
+    ];
+
+    $block_categories['#type'] = 'container';
+    $block_categories['#attributes']['class'][] = 'block-categories';
+    $block_categories['#attributes']['class'][] = 'js-layout-builder-categories';
+    $block_categories['#attributes']['data-layout-builder-target-highlight-id'] = $this->blockAddHighlightId($delta, $region);
+
+    $definitions = $this->blockManager->getFilteredDefinitions('layout_builder', $this->getPopulatedContexts($section_storage), [
+      'section_storage' => $section_storage,
+      'delta' => $delta,
+      'region' => $region,
+    ]);
+    $grouped_definitions = $this->blockManager->getGroupedDefinitions($definitions);
+    foreach ($grouped_definitions as $category => $blocks) {
+      $block_categories[$category]['#type'] = 'details';
+      $block_categories[$category]['#attributes']['class'][] = 'js-layout-builder-category';
+      $block_categories[$category]['#open'] = TRUE;
+      $block_categories[$category]['#title'] = $category;
+      $block_categories[$category]['links'] = $this->getBlockLinks($section_storage, $delta, $region, $blocks);
+    }
+    $build['block_categories'] = $block_categories;
+    return $build;
+  }
+
+  /**
+   * Provides the UI for choosing a new inline block.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   * @param int $delta
+   *   The delta of the section to splice.
+   * @param string $region
+   *   The region the block is going in.
+   *
+   * @return array
+   *   A render array.
+   */
+  public function inlineBlockList(SectionStorageInterface $section_storage, int $delta, $region) {
+    $definitions = $this->blockManager->getFilteredDefinitions('layout_builder', $this->getPopulatedContexts($section_storage), [
+      'section_storage' => $section_storage,
+      'region' => $region,
+      'list' => 'inline_blocks',
+    ]);
+    $blocks = $this->blockManager->getGroupedDefinitions($definitions);
+    $build = [];
+    $inline_blocks_category = (string) $this->t('Inline blocks');
+    if (isset($blocks[$inline_blocks_category])) {
+      $build['links'] = $this->getBlockLinks($section_storage, $delta, $region, $blocks[$inline_blocks_category]);
+      $build['links']['#attributes']['class'][] = 'inline-block-list';
+      foreach ($build['links']['#links'] as &$link) {
+        $link['attributes']['class'][] = 'inline-block-list__item';
+      }
+      $build['back_button'] = [
+        '#type' => 'link',
+        '#url' => Url::fromRoute('layout_builder.choose_block',
+          [
+            'section_storage_type' => $section_storage->getStorageType(),
+            'section_storage' => $section_storage->getStorageId(),
+            'delta' => $delta,
+            'region' => $region,
+          ]
+        ),
+        '#title' => $this->t('Back'),
+        '#attributes' => $this->getAjaxAttributes(),
+      ];
+    }
+    $build['links']['#attributes']['data-layout-builder-target-highlight-id'] = $this->blockAddHighlightId($delta, $region);
+    return $build;
+  }
+
+  /**
+   * Gets a render array of block links.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   * @param int $delta
+   *   The delta of the section to splice.
+   * @param string $region
+   *   The region the block is going in.
+   * @param array $blocks
+   *   The information for each block.
+   *
+   * @return array
+   *   The block links render array.
+   */
+  protected function getBlockLinks(SectionStorageInterface $section_storage, int $delta, $region, array $blocks) {
+    $links = [];
+    foreach ($blocks as $block_id => $block) {
+      $attributes = $this->getAjaxAttributes();
+      $attributes['class'][] = 'js-layout-builder-block-link';
+      $link = [
+        'title' => $block['admin_label'],
+        'url' => Url::fromRoute('layout_builder.add_block',
+          [
+            'section_storage_type' => $section_storage->getStorageType(),
+            'section_storage' => $section_storage->getStorageId(),
+            'delta' => $delta,
+            'region' => $region,
+            'plugin_id' => $block_id,
+          ]
+        ),
+        'attributes' => $attributes,
+      ];
+
+      $links[] = $link;
+    }
+    return [
+      '#theme' => 'links',
+      '#links' => $links,
+    ];
+  }
+
+  /**
+   * Get dialog attributes if an ajax request.
+   *
+   * @return array
+   *   The attributes array.
+   */
+  protected function getAjaxAttributes() {
+    if ($this->isAjax()) {
+      return [
+        'class' => ['use-ajax'],
+        'data-dialog-type' => 'dialog',
+        'data-dialog-renderer' => 'off_canvas',
+      ];
+    }
+    return [];
+  }
+
+}

core/modules/layout_builder/src/Controller/ChooseSectionController.php

@@ -0,0 +1,111 @@
+<?php
+
+namespace Drupal\layout_builder\Controller;
+
+use Drupal\Core\Ajax\AjaxHelperTrait;
+use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
+use Drupal\Core\Layout\LayoutPluginManagerInterface;
+use Drupal\Core\Plugin\PluginFormInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\Core\Url;
+use Drupal\layout_builder\Context\LayoutBuilderContextTrait;
+use Drupal\layout_builder\LayoutBuilderHighlightTrait;
+use Drupal\layout_builder\SectionStorageInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Defines a controller to choose a new section.
+ *
+ * @internal
+ *   Controller classes are internal.
+ */
+class ChooseSectionController implements ContainerInjectionInterface {
+
+  use AjaxHelperTrait;
+  use LayoutBuilderContextTrait;
+  use LayoutBuilderHighlightTrait;
+  use StringTranslationTrait;
+
+  /**
+   * The layout manager.
+   *
+   * @var \Drupal\Core\Layout\LayoutPluginManagerInterface
+   */
+  protected $layoutManager;
+
+  /**
+   * ChooseSectionController constructor.
+   *
+   * @param \Drupal\Core\Layout\LayoutPluginManagerInterface $layout_manager
+   *   The layout manager.
+   */
+  public function __construct(LayoutPluginManagerInterface $layout_manager) {
+    $this->layoutManager = $layout_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('plugin.manager.core.layout')
+    );
+  }
+
+  /**
+   * Choose a layout plugin to add as a section.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   * @param int $delta
+   *   The delta of the section to splice.
+   *
+   * @return array
+   *   The render array.
+   */
+  public function build(SectionStorageInterface $section_storage, int $delta) {
+    $items = [];
+    $definitions = $this->layoutManager->getFilteredDefinitions('layout_builder', $this->getPopulatedContexts($section_storage), ['section_storage' => $section_storage]);
+    foreach ($definitions as $plugin_id => $definition) {
+      $layout = $this->layoutManager->createInstance($plugin_id);
+      $item = [
+        '#type' => 'link',
+        '#title' => [
+          'icon' => $definition->getIcon(60, 80, 1, 3),
+          'label' => [
+            '#type' => 'container',
+            '#children' => $definition->getLabel(),
+          ],
+        ],
+        '#url' => Url::fromRoute(
+          $layout instanceof PluginFormInterface ? 'layout_builder.configure_section' : 'layout_builder.add_section',
+          [
+            'section_storage_type' => $section_storage->getStorageType(),
+            'section_storage' => $section_storage->getStorageId(),
+            'delta' => $delta,
+            'plugin_id' => $plugin_id,
+          ]
+        ),
+      ];
+      if ($this->isAjax()) {
+        $item['#attributes']['class'][] = 'use-ajax';
+        $item['#attributes']['data-dialog-type'][] = 'dialog';
+        $item['#attributes']['data-dialog-renderer'][] = 'off_canvas';
+      }
+      $items[$plugin_id] = $item;
+    }
+    $output['layouts'] = [
+      '#theme' => 'item_list__layouts',
+      '#items' => $items,
+      '#attributes' => [
+        'class' => [
+          'layout-selection',
+        ],
+        'data-layout-builder-target-highlight-id' => $this->sectionAddHighlightId($delta),
+      ],
+    ];
+
+    return $output;
+  }
+
+}

core/modules/layout_builder/src/Controller/LayoutBuilderController.php

@@ -0,0 +1,49 @@
+<?php
+
+namespace Drupal\layout_builder\Controller;
+
+use Drupal\Component\Assertion\Inspector;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\layout_builder\SectionStorageInterface;
+
+/**
+ * Defines a controller to provide the Layout Builder admin UI.
+ *
+ * @internal
+ *   Controller classes are internal.
+ */
+class LayoutBuilderController {
+
+  use StringTranslationTrait;
+
+  /**
+   * Provides a title callback.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   *
+   * @return string
+   *   The title for the layout page.
+   */
+  public function title(SectionStorageInterface $section_storage) {
+    assert(Inspector::assertStringable($section_storage->label()), 'Section storage label is expected to be a string.');
+    return $this->t('Edit layout for %label', ['%label' => $section_storage->label() ?? $section_storage->getStorageType() . ' ' . $section_storage->getStorageId()]);
+  }
+
+  /**
+   * Renders the Layout UI.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   *
+   * @return array
+   *   A render array.
+   */
+  public function layout(SectionStorageInterface $section_storage) {
+    return [
+      '#type' => 'layout_builder',
+      '#section_storage' => $section_storage,
+    ];
+  }
+
+}

core/modules/layout_builder/src/Controller/LayoutBuilderHtmlEntityFormController.php

@@ -0,0 +1,73 @@
+<?php
+
+namespace Drupal\layout_builder\Controller;
+
+use Drupal\Component\Utility\NestedArray;
+use Drupal\Core\Controller\FormController;
+use Drupal\Core\DependencyInjection\DependencySerializationTrait;
+use Drupal\Core\Routing\RouteMatchInterface;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Overrides the entity form controller service for layout builder operations.
+ */
+class LayoutBuilderHtmlEntityFormController extends FormController {
+
+  use DependencySerializationTrait;
+
+  /**
+   * The entity form controller being decorated.
+   *
+   * @var \Drupal\Core\Controller\FormController
+   */
+  protected $entityFormController;
+
+  /**
+   * Constructs a LayoutBuilderHtmlEntityFormController object.
+   *
+   * @param \Drupal\Core\Controller\FormController $entity_form_controller
+   *   The entity form controller being decorated.
+   */
+  public function __construct(FormController $entity_form_controller) {
+    $this->entityFormController = $entity_form_controller;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getContentResult(Request $request, RouteMatchInterface $route_match) {
+    $form = $this->entityFormController->getContentResult($request, $route_match);
+
+    // If the form render element has a #layout_builder_element_keys property,
+    // first set the form element as a child of the root render array. Use the
+    // keys to get the layout builder element from the form render array and
+    // copy it to a separate child element of the root element to prevent any
+    // forms within the layout builder element from being nested.
+    if (isset($form['#layout_builder_element_keys'])) {
+      $build['form'] = &$form;
+      $layout_builder_element = &NestedArray::getValue($form, $form['#layout_builder_element_keys']);
+      $build['layout_builder'] = $layout_builder_element;
+      // Remove the layout builder element within the form.
+      $layout_builder_element = [];
+      return $build;
+    }
+
+    // If no #layout_builder_element_keys property, return form as is.
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getFormArgument(RouteMatchInterface $route_match) {
+    return $this->entityFormController->getFormArgument($route_match);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getFormObject(RouteMatchInterface $route_match, $form_arg) {
+    return $this->entityFormController->getFormObject($route_match, $form_arg);
+  }
+
+}

core/modules/layout_builder/src/Controller/LayoutRebuildTrait.php

@@ -0,0 +1,51 @@
+<?php
+
+namespace Drupal\layout_builder\Controller;
+
+use Drupal\Core\Ajax\AjaxResponse;
+use Drupal\Core\Ajax\CloseDialogCommand;
+use Drupal\Core\Ajax\ReplaceCommand;
+use Drupal\layout_builder\SectionStorageInterface;
+
+/**
+ * Provides AJAX responses to rebuild the Layout Builder.
+ */
+trait LayoutRebuildTrait {
+
+  /**
+   * Rebuilds the layout.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   *
+   * @return \Drupal\Core\Ajax\AjaxResponse
+   *   An AJAX response to either rebuild the layout and close the dialog, or
+   *   reload the page.
+   */
+  protected function rebuildAndClose(SectionStorageInterface $section_storage) {
+    $response = $this->rebuildLayout($section_storage);
+    $response->addCommand(new CloseDialogCommand('#drupal-off-canvas'));
+    return $response;
+  }
+
+  /**
+   * Rebuilds the layout.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   *
+   * @return \Drupal\Core\Ajax\AjaxResponse
+   *   An AJAX response to either rebuild the layout and close the dialog, or
+   *   reload the page.
+   */
+  protected function rebuildLayout(SectionStorageInterface $section_storage) {
+    $response = new AjaxResponse();
+    $layout = [
+      '#type' => 'layout_builder',
+      '#section_storage' => $section_storage,
+    ];
+    $response->addCommand(new ReplaceCommand('#layout-builder', $layout));
+    return $response;
+  }
+
+}

core/modules/layout_builder/src/Controller/MoveBlockController.php

@@ -0,0 +1,91 @@
+<?php
+
+namespace Drupal\layout_builder\Controller;
+
+use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
+use Drupal\layout_builder\LayoutTempstoreRepositoryInterface;
+use Drupal\layout_builder\SectionStorageInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Defines a controller to move a block.
+ *
+ * @internal
+ *   Controller classes are internal.
+ */
+class MoveBlockController implements ContainerInjectionInterface {
+
+  use LayoutRebuildTrait;
+
+  /**
+   * The layout tempstore repository.
+   *
+   * @var \Drupal\layout_builder\LayoutTempstoreRepositoryInterface
+   */
+  protected $layoutTempstoreRepository;
+
+  /**
+   * LayoutController constructor.
+   *
+   * @param \Drupal\layout_builder\LayoutTempstoreRepositoryInterface $layout_tempstore_repository
+   *   The layout tempstore repository.
+   */
+  public function __construct(LayoutTempstoreRepositoryInterface $layout_tempstore_repository) {
+    $this->layoutTempstoreRepository = $layout_tempstore_repository;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('layout_builder.tempstore_repository')
+    );
+  }
+
+  /**
+   * Moves a block to another region.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   * @param int $delta_from
+   *   The delta of the original section.
+   * @param int $delta_to
+   *   The delta of the destination section.
+   * @param string $region_to
+   *   The new region for this block.
+   * @param string $block_uuid
+   *   The UUID for this block.
+   * @param string|null $preceding_block_uuid
+   *   (optional) If provided, the UUID of the block to insert this block after.
+   *
+   * @return \Drupal\Core\Ajax\AjaxResponse
+   *   An AJAX response.
+   */
+  public function build(SectionStorageInterface $section_storage, int $delta_from, int $delta_to, $region_to, $block_uuid, $preceding_block_uuid = NULL) {
+    $section = $section_storage->getSection($delta_from);
+
+    $component = $section->getComponent($block_uuid);
+    $section->removeComponent($block_uuid);
+
+    // If the block is moving from one section to another, update the original
+    // section and load the new one.
+    if ($delta_from !== $delta_to) {
+      $section = $section_storage->getSection($delta_to);
+    }
+
+    // If a preceding block was specified, insert after that. Otherwise add the
+    // block to the front.
+    $component->setRegion($region_to);
+    if (isset($preceding_block_uuid)) {
+      $section->insertAfterComponent($preceding_block_uuid, $component);
+    }
+    else {
+      $section->insertComponent(0, $component);
+    }
+
+    $this->layoutTempstoreRepository->set($section_storage);
+    return $this->rebuildLayout($section_storage);
+  }
+
+}

core/modules/layout_builder/src/DefaultsSectionStorageInterface.php

@@ -0,0 +1,10 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+use Drupal\Core\Config\Entity\ThirdPartySettingsInterface;
+
+/**
+ * Defines an interface for an object that stores layout sections for defaults.
+ */
+interface DefaultsSectionStorageInterface extends SectionStorageInterface, ThirdPartySettingsInterface, LayoutBuilderEnabledInterface, LayoutBuilderOverridableInterface {}

core/modules/layout_builder/src/Element/LayoutBuilder.php

@@ -0,0 +1,382 @@
+<?php
+
+namespace Drupal\layout_builder\Element;
+
+use Drupal\Core\Ajax\AjaxHelperTrait;
+use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
+use Drupal\Core\Plugin\PluginFormInterface;
+use Drupal\Core\Render\Attribute\RenderElement;
+use Drupal\Core\Render\Element;
+use Drupal\Core\Render\Element\RenderElementBase;
+use Drupal\Core\Url;
+use Drupal\layout_builder\Context\LayoutBuilderContextTrait;
+use Drupal\layout_builder\Event\PrepareLayoutEvent;
+use Drupal\layout_builder\LayoutBuilderEvents;
+use Drupal\layout_builder\LayoutBuilderHighlightTrait;
+use Drupal\layout_builder\SectionStorageInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+use Symfony\Contracts\EventDispatcher\EventDispatcherInterface;
+
+/**
+ * Defines a render element for building the Layout Builder UI.
+ *
+ * @internal
+ *   Plugin classes are internal.
+ */
+#[RenderElement('layout_builder')]
+class LayoutBuilder extends RenderElementBase implements ContainerFactoryPluginInterface {
+
+  use AjaxHelperTrait;
+  use LayoutBuilderContextTrait;
+  use LayoutBuilderHighlightTrait;
+
+  /**
+   * The event dispatcher.
+   *
+   * @var \Symfony\Contracts\EventDispatcher\EventDispatcherInterface
+   */
+  protected $eventDispatcher;
+
+  /**
+   * Constructs a new LayoutBuilder.
+   *
+   * @param array $configuration
+   *   A configuration array containing information about the plugin instance.
+   * @param string $plugin_id
+   *   The plugin ID for the plugin instance.
+   * @param mixed $plugin_definition
+   *   The plugin implementation definition.
+   * @param \Symfony\Contracts\EventDispatcher\EventDispatcherInterface $event_dispatcher
+   *   The event dispatcher service.
+   */
+  public function __construct(array $configuration, $plugin_id, $plugin_definition, EventDispatcherInterface $event_dispatcher) {
+    parent::__construct($configuration, $plugin_id, $plugin_definition);
+    $this->eventDispatcher = $event_dispatcher;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
+    return new static(
+      $configuration,
+      $plugin_id,
+      $plugin_definition,
+      $container->get('event_dispatcher')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getInfo() {
+    return [
+      '#section_storage' => NULL,
+      '#pre_render' => [
+        [$this, 'preRender'],
+      ],
+    ];
+  }
+
+  /**
+   * Pre-render callback: Renders the Layout Builder UI.
+   */
+  public function preRender($element) {
+    if ($element['#section_storage'] instanceof SectionStorageInterface) {
+      $element['layout_builder'] = $this->layout($element['#section_storage']);
+    }
+    return $element;
+  }
+
+  /**
+   * Renders the Layout UI.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   *
+   * @return array
+   *   A render array.
+   */
+  protected function layout(SectionStorageInterface $section_storage) {
+    $this->prepareLayout($section_storage);
+
+    $output = [];
+    if ($this->isAjax()) {
+      $output['status_messages'] = [
+        '#type' => 'status_messages',
+      ];
+    }
+    $count = 0;
+    for ($i = 0; $i < $section_storage->count(); $i++) {
+      $output[] = $this->buildAddSectionLink($section_storage, $count);
+      $output[] = $this->buildAdministrativeSection($section_storage, $count);
+      $count++;
+    }
+    $output[] = $this->buildAddSectionLink($section_storage, $count);
+    $output['#attached']['library'][] = 'layout_builder/drupal.layout_builder';
+    // As the Layout Builder UI is typically displayed using the frontend theme,
+    // it is not marked as an administrative page at the route level even though
+    // it performs an administrative task. Mark this as an administrative page
+    // for JavaScript.
+    $output['#attached']['drupalSettings']['path']['currentPathIsAdmin'] = TRUE;
+    $output['#type'] = 'container';
+    $output['#attributes']['id'] = 'layout-builder';
+    $output['#attributes']['class'][] = 'layout-builder';
+    // Mark this UI as uncacheable.
+    $output['#cache']['max-age'] = 0;
+    return $output;
+  }
+
+  /**
+   * Prepares a layout for use in the UI.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   */
+  protected function prepareLayout(SectionStorageInterface $section_storage) {
+    $event = new PrepareLayoutEvent($section_storage);
+    $this->eventDispatcher->dispatch($event, LayoutBuilderEvents::PREPARE_LAYOUT);
+  }
+
+  /**
+   * Builds a link to add a new section at a given delta.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   * @param int $delta
+   *   The delta of the section to splice.
+   *
+   * @return array
+   *   A render array for a link.
+   */
+  protected function buildAddSectionLink(SectionStorageInterface $section_storage, $delta) {
+    $storage_type = $section_storage->getStorageType();
+    $storage_id = $section_storage->getStorageId();
+
+    // If the delta and the count are the same, it is either the end of the
+    // layout or an empty layout.
+    if ($delta === count($section_storage)) {
+      if ($delta === 0) {
+        $title = $this->t('Add section');
+      }
+      else {
+        $title = $this->t('Add section <span class="visually-hidden">at end of layout</span>');
+      }
+    }
+    // If the delta and the count are different, it is either the beginning of
+    // the layout or in between two sections.
+    else {
+      if ($delta === 0) {
+        $title = $this->t('Add section <span class="visually-hidden">at start of layout</span>');
+      }
+      else {
+        $title = $this->t('Add section <span class="visually-hidden">between @first and @second</span>', ['@first' => $delta, '@second' => $delta + 1]);
+      }
+    }
+
+    return [
+      'link' => [
+        '#type' => 'link',
+        '#title' => $title,
+        '#url' => Url::fromRoute('layout_builder.choose_section',
+          [
+            'section_storage_type' => $storage_type,
+            'section_storage' => $storage_id,
+            'delta' => $delta,
+          ],
+          [
+            'attributes' => [
+              'class' => [
+                'use-ajax',
+                'layout-builder__link',
+                'layout-builder__link--add',
+              ],
+              'data-dialog-type' => 'dialog',
+              'data-dialog-renderer' => 'off_canvas',
+            ],
+          ]
+        ),
+      ],
+      '#type' => 'container',
+      '#attributes' => [
+        'class' => ['layout-builder__add-section'],
+        'data-layout-builder-highlight-id' => $this->sectionAddHighlightId($delta),
+      ],
+    ];
+  }
+
+  /**
+   * Builds the render array for the layout section while editing.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   * @param int $delta
+   *   The delta of the section.
+   *
+   * @return array
+   *   The render array for a given section.
+   */
+  protected function buildAdministrativeSection(SectionStorageInterface $section_storage, $delta) {
+    $storage_type = $section_storage->getStorageType();
+    $storage_id = $section_storage->getStorageId();
+    $section = $section_storage->getSection($delta);
+
+    $layout = $section->getLayout($this->getPopulatedContexts($section_storage));
+    $layout_settings = $section->getLayoutSettings();
+    $section_label = !empty($layout_settings['label']) ? $layout_settings['label'] : $this->t('Section @section', ['@section' => $delta + 1]);
+
+    $build = $section->toRenderArray($this->getPopulatedContexts($section_storage), TRUE);
+    $layout_definition = $layout->getPluginDefinition();
+
+    $region_labels = $layout_definition->getRegionLabels();
+    foreach ($layout_definition->getRegions() as $region => $info) {
+      if (!empty($build[$region])) {
+        foreach (Element::children($build[$region]) as $uuid) {
+          $build[$region][$uuid]['#attributes']['class'][] = 'js-layout-builder-block';
+          $build[$region][$uuid]['#attributes']['class'][] = 'layout-builder-block';
+          $build[$region][$uuid]['#attributes']['data-layout-block-uuid'] = $uuid;
+          $build[$region][$uuid]['#attributes']['data-layout-builder-highlight-id'] = $this->blockUpdateHighlightId($uuid);
+          $build[$region][$uuid]['#contextual_links'] = [
+            'layout_builder_block' => [
+              'route_parameters' => [
+                'section_storage_type' => $storage_type,
+                'section_storage' => $storage_id,
+                'delta' => $delta,
+                'region' => $region,
+                'uuid' => $uuid,
+              ],
+              // Add metadata about the current operations available in
+              // contextual links. This will invalidate the client-side cache of
+              // links that were cached before the 'move' link was added.
+              // @see layout_builder.links.contextual.yml
+              'metadata' => [
+                'operations' => 'move:update:remove',
+              ],
+            ],
+          ];
+        }
+      }
+
+      $build[$region]['layout_builder_add_block']['link'] = [
+        '#type' => 'link',
+        // Add one to the current delta since it is zero-indexed.
+        '#title' => $this->t('Add block <span class="visually-hidden">in @section, @region region</span>', ['@section' => $section_label, '@region' => $region_labels[$region]]),
+        '#url' => Url::fromRoute('layout_builder.choose_block',
+          [
+            'section_storage_type' => $storage_type,
+            'section_storage' => $storage_id,
+            'delta' => $delta,
+            'region' => $region,
+          ],
+          [
+            'attributes' => [
+              'class' => [
+                'use-ajax',
+                'layout-builder__link',
+                'layout-builder__link--add',
+              ],
+              'data-dialog-type' => 'dialog',
+              'data-dialog-renderer' => 'off_canvas',
+            ],
+          ]
+        ),
+      ];
+      $build[$region]['layout_builder_add_block']['#type'] = 'container';
+      $build[$region]['layout_builder_add_block']['#attributes'] = [
+        'class' => ['layout-builder__add-block'],
+        'data-layout-builder-highlight-id' => $this->blockAddHighlightId($delta, $region),
+      ];
+      $build[$region]['layout_builder_add_block']['#weight'] = 1000;
+      $build[$region]['#attributes']['data-region'] = $region;
+      $build[$region]['#attributes']['class'][] = 'layout-builder__region';
+      $build[$region]['#attributes']['class'][] = 'js-layout-builder-region';
+      $build[$region]['#attributes']['role'] = 'group';
+      $build[$region]['#attributes']['aria-label'] = $this->t('@region region in @section', [
+        '@region' => $info['label'],
+        '@section' => $section_label,
+      ]);
+
+      // Get weights of all children for use by the region label.
+      $weights = array_map(function ($a) {
+        return $a['#weight'] ?? 0;
+      }, $build[$region]);
+
+      // The region label is made visible when the move block dialog is open.
+      $build[$region]['region_label'] = [
+        '#type' => 'container',
+        '#attributes' => [
+          'class' => ['layout__region-info', 'layout-builder__region-label'],
+          // A more detailed version of this information is already read by
+          // screen readers, so this label can be hidden from them.
+          'aria-hidden' => TRUE,
+        ],
+        '#markup' => $this->t('Region: @region', ['@region' => $info['label']]),
+        // Ensures the region label is displayed first.
+        '#weight' => min($weights) - 1,
+      ];
+    }
+
+    $build['#attributes']['data-layout-update-url'] = Url::fromRoute('layout_builder.move_block', [
+      'section_storage_type' => $storage_type,
+      'section_storage' => $storage_id,
+    ])->toString();
+
+    $build['#attributes']['data-layout-delta'] = $delta;
+    $build['#attributes']['class'][] = 'layout-builder__layout';
+    $build['#attributes']['data-layout-builder-highlight-id'] = $this->sectionUpdateHighlightId($delta);
+
+    return [
+      '#type' => 'container',
+      '#attributes' => [
+        'class' => ['layout-builder__section'],
+        'role' => 'group',
+        'aria-label' => $section_label,
+      ],
+      'remove' => [
+        '#type' => 'link',
+        '#title' => $this->t('Remove @section', ['@section' => $section_label]),
+        '#url' => Url::fromRoute('layout_builder.remove_section', [
+          'section_storage_type' => $storage_type,
+          'section_storage' => $storage_id,
+          'delta' => $delta,
+        ]),
+        '#attributes' => [
+          'class' => [
+            'use-ajax',
+            'layout-builder__link',
+            'layout-builder__link--remove',
+          ],
+          'data-dialog-type' => 'dialog',
+          'data-dialog-renderer' => 'off_canvas',
+        ],
+      ],
+      // The section label is added to sections without a "Configure section"
+      // link, and is only visible when the move block dialog is open.
+      'section_label' => [
+        '#markup' => $this->t('<span class="layout-builder__section-label" aria-hidden="true">@section</span>', ['@section' => $section_label]),
+        '#access' => !$layout instanceof PluginFormInterface,
+      ],
+      'configure' => [
+        '#type' => 'link',
+        '#title' => $this->t('Configure @section', ['@section' => $section_label]),
+        '#access' => $layout instanceof PluginFormInterface,
+        '#url' => Url::fromRoute('layout_builder.configure_section', [
+          'section_storage_type' => $storage_type,
+          'section_storage' => $storage_id,
+          'delta' => $delta,
+        ]),
+        '#attributes' => [
+          'class' => [
+            'use-ajax',
+            'layout-builder__link',
+            'layout-builder__link--configure',
+          ],
+          'data-dialog-type' => 'dialog',
+          'data-dialog-renderer' => 'off_canvas',
+        ],
+      ],
+      'layout-builder__section' => $build,
+    ];
+  }
+
+}

core/modules/layout_builder/src/Entity/LayoutBuilderEntityViewDisplay.php

@@ -0,0 +1,529 @@
+<?php
+
+namespace Drupal\layout_builder\Entity;
+
+use Drupal\Component\Plugin\ConfigurableInterface;
+use Drupal\Component\Plugin\DerivativeInspectionInterface;
+use Drupal\Component\Plugin\PluginBase;
+use Drupal\Core\Cache\CacheableMetadata;
+use Drupal\Core\Entity\Entity\EntityViewDisplay as BaseEntityViewDisplay;
+use Drupal\Core\Entity\EntityStorageInterface;
+use Drupal\Core\Entity\FieldableEntityInterface;
+use Drupal\Core\Plugin\Context\Context;
+use Drupal\Core\Plugin\Context\ContextDefinition;
+use Drupal\Core\Plugin\Context\EntityContext;
+use Drupal\Core\Render\Element;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\field\Entity\FieldConfig;
+use Drupal\field\Entity\FieldStorageConfig;
+use Drupal\layout_builder\LayoutEntityHelperTrait;
+use Drupal\layout_builder\Plugin\SectionStorage\OverridesSectionStorage;
+use Drupal\layout_builder\Section;
+use Drupal\layout_builder\SectionComponent;
+use Drupal\layout_builder\SectionListTrait;
+
+/**
+ * Provides an entity view display entity that has a layout.
+ */
+class LayoutBuilderEntityViewDisplay extends BaseEntityViewDisplay implements LayoutEntityDisplayInterface {
+
+  use LayoutEntityHelperTrait;
+  use SectionListTrait;
+
+  /**
+   * The entity field manager.
+   *
+   * @var \Drupal\Core\Entity\EntityFieldManagerInterface
+   */
+  protected $entityFieldManager;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function __construct(array $values, $entity_type) {
+    // Set $entityFieldManager before calling the parent constructor because the
+    // constructor will call init() which then calls setComponent() which needs
+    // $entityFieldManager.
+    $this->entityFieldManager = \Drupal::service('entity_field.manager');
+    parent::__construct($values, $entity_type);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function isOverridable() {
+    return $this->isLayoutBuilderEnabled() && $this->getThirdPartySetting('layout_builder', 'allow_custom', FALSE);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setOverridable($overridable = TRUE) {
+    $this->setThirdPartySetting('layout_builder', 'allow_custom', $overridable);
+    // Enable Layout Builder if it's not already enabled and overriding.
+    if ($overridable && !$this->isLayoutBuilderEnabled()) {
+      $this->enableLayoutBuilder();
+    }
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function isLayoutBuilderEnabled() {
+    // Layout Builder must not be enabled for the '_custom' view mode that is
+    // used for on-the-fly rendering of fields in isolation from the entity.
+    if ($this->isCustomMode()) {
+      return FALSE;
+    }
+    return (bool) $this->getThirdPartySetting('layout_builder', 'enabled');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function enableLayoutBuilder() {
+    $this->setThirdPartySetting('layout_builder', 'enabled', TRUE);
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function disableLayoutBuilder() {
+    $this->setOverridable(FALSE);
+    $this->setThirdPartySetting('layout_builder', 'enabled', FALSE);
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getSections() {
+    return $this->getThirdPartySetting('layout_builder', 'sections', []);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setSections(array $sections) {
+    // Third-party settings must be completely unset instead of stored as an
+    // empty array.
+    if (!$sections) {
+      $this->unsetThirdPartySetting('layout_builder', 'sections');
+    }
+    else {
+      $this->setThirdPartySetting('layout_builder', 'sections', array_values($sections));
+    }
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function preSave(EntityStorageInterface $storage) {
+
+    $original_value = isset($this->original) ? $this->original->isOverridable() : FALSE;
+    $new_value = $this->isOverridable();
+    if ($original_value !== $new_value) {
+      $entity_type_id = $this->getTargetEntityTypeId();
+      $bundle = $this->getTargetBundle();
+
+      if ($new_value) {
+        $this->addSectionField($entity_type_id, $bundle, OverridesSectionStorage::FIELD_NAME);
+      }
+      else {
+        $this->removeSectionField($entity_type_id, $bundle, OverridesSectionStorage::FIELD_NAME);
+      }
+    }
+
+    parent::preSave($storage);
+
+    $already_enabled = isset($this->original) ? $this->original->isLayoutBuilderEnabled() : FALSE;
+    $set_enabled = $this->isLayoutBuilderEnabled();
+    if ($already_enabled !== $set_enabled) {
+      if ($set_enabled) {
+        // Loop through all existing field-based components and add them as
+        // section-based components.
+        $components = $this->getComponents();
+        // Sort the components by weight.
+        uasort($components, 'Drupal\Component\Utility\SortArray::sortByWeightElement');
+        foreach ($components as $name => $component) {
+          $this->setComponent($name, $component);
+        }
+      }
+      else {
+        // When being disabled, remove all existing section data.
+        $this->removeAllSections();
+      }
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function save(): int {
+    $return = parent::save();
+    if (!\Drupal::moduleHandler()->moduleExists('layout_builder_expose_all_field_blocks')) {
+      // Invalidate the block cache in order to regenerate field block
+      // definitions.
+      \Drupal::service('plugin.manager.block')->clearCachedDefinitions();
+    }
+    return $return;
+  }
+
+  /**
+   * Removes a layout section field if it is no longer needed.
+   *
+   * Because the field is shared across all view modes, the field will only be
+   * removed if no other view modes are using it.
+   *
+   * @param string $entity_type_id
+   *   The entity type ID.
+   * @param string $bundle
+   *   The bundle.
+   * @param string $field_name
+   *   The name for the layout section field.
+   */
+  protected function removeSectionField($entity_type_id, $bundle, $field_name) {
+    /** @var \Drupal\Core\Config\Entity\ConfigEntityStorageInterface $storage */
+    $storage = $this->entityTypeManager()->getStorage($this->getEntityTypeId());
+    $query = $storage->getQuery()
+      ->condition('targetEntityType', $this->getTargetEntityTypeId())
+      ->condition('bundle', $this->getTargetBundle())
+      ->condition('mode', $this->getMode(), '<>')
+      ->condition('third_party_settings.layout_builder.allow_custom', TRUE);
+    $enabled = (bool) $query->count()->execute();
+    if (!$enabled && $field = FieldConfig::loadByName($entity_type_id, $bundle, $field_name)) {
+      $field->delete();
+    }
+  }
+
+  /**
+   * Adds a layout section field to a given bundle.
+   *
+   * @param string $entity_type_id
+   *   The entity type ID.
+   * @param string $bundle
+   *   The bundle.
+   * @param string $field_name
+   *   The name for the layout section field.
+   */
+  protected function addSectionField($entity_type_id, $bundle, $field_name) {
+    $field = FieldConfig::loadByName($entity_type_id, $bundle, $field_name);
+    if (!$field) {
+      $field_storage = FieldStorageConfig::loadByName($entity_type_id, $field_name);
+      if (!$field_storage) {
+        $field_storage = FieldStorageConfig::create([
+          'entity_type' => $entity_type_id,
+          'field_name' => $field_name,
+          'type' => 'layout_section',
+          'locked' => TRUE,
+        ]);
+        $field_storage->setTranslatable(FALSE);
+        $field_storage->save();
+      }
+
+      $field = FieldConfig::create([
+        'field_storage' => $field_storage,
+        'bundle' => $bundle,
+        'label' => t('Layout'),
+      ]);
+      $field->setTranslatable(FALSE);
+      $field->save();
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function createCopy($mode) {
+    // Disable Layout Builder and remove any sections copied from the original.
+    return parent::createCopy($mode)
+      ->setSections([])
+      ->disableLayoutBuilder();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getDefaultRegion() {
+    if ($this->hasSection(0)) {
+      return $this->getSection(0)->getDefaultRegion();
+    }
+
+    return parent::getDefaultRegion();
+  }
+
+  /**
+   * Wraps the context repository service.
+   *
+   * @return \Drupal\Core\Plugin\Context\ContextRepositoryInterface
+   *   The context repository service.
+   */
+  protected function contextRepository() {
+    return \Drupal::service('context.repository');
+  }
+
+  /**
+   * Indicates if this display is using the '_custom' view mode.
+   *
+   * @return bool
+   *   TRUE if this display is using the '_custom' view mode, FALSE otherwise.
+   */
+  protected function isCustomMode() {
+    return $this->getOriginalMode() === static::CUSTOM_MODE;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildMultiple(array $entities) {
+    $build_list = parent::buildMultiple($entities);
+
+    // Layout Builder can not be enabled for the '_custom' view mode that is
+    // used for on-the-fly rendering of fields in isolation from the entity.
+    if ($this->isCustomMode()) {
+      return $build_list;
+    }
+
+    foreach ($entities as $id => $entity) {
+      $build_list[$id]['_layout_builder'] = $this->buildSections($entity);
+
+      // If there are any sections, remove all fields with configurable display
+      // from the existing build. These fields are replicated within sections as
+      // field blocks by ::setComponent().
+      if (!Element::isEmpty($build_list[$id]['_layout_builder'])) {
+        foreach ($build_list[$id] as $name => $build_part) {
+          $field_definition = $this->getFieldDefinition($name);
+          if ($field_definition && $field_definition->isDisplayConfigurable($this->displayContext)) {
+            unset($build_list[$id][$name]);
+          }
+        }
+      }
+    }
+
+    return $build_list;
+  }
+
+  /**
+   * Builds the render array for the sections of a given entity.
+   *
+   * @param \Drupal\Core\Entity\FieldableEntityInterface $entity
+   *   The entity.
+   *
+   * @return array
+   *   The render array representing the sections of the entity.
+   */
+  protected function buildSections(FieldableEntityInterface $entity) {
+    $contexts = $this->getContextsForEntity($entity);
+    $label = new TranslatableMarkup('@entity being viewed', [
+      '@entity' => $entity->getEntityType()->getSingularLabel(),
+    ]);
+    $contexts['layout_builder.entity'] = EntityContext::fromEntity($entity, $label);
+
+    $cacheability = new CacheableMetadata();
+    $storage = $this->sectionStorageManager()->findByContext($contexts, $cacheability);
+
+    $build = [];
+    if ($storage) {
+      foreach ($storage->getSections() as $delta => $section) {
+        $build[$delta] = $section->toRenderArray($contexts);
+      }
+    }
+    // The render array is built based on decisions made by SectionStorage
+    // plugins and therefore it needs to depend on the accumulated
+    // cacheability of those decisions.
+    $cacheability->applyTo($build);
+    return $build;
+  }
+
+  /**
+   * Gets the available contexts for a given entity.
+   *
+   * @param \Drupal\Core\Entity\FieldableEntityInterface $entity
+   *   The entity.
+   *
+   * @return \Drupal\Core\Plugin\Context\ContextInterface[]
+   *   An array of context objects for a given entity.
+   */
+  protected function getContextsForEntity(FieldableEntityInterface $entity) {
+    $available_context_ids = array_keys($this->contextRepository()->getAvailableContexts());
+    return [
+      'view_mode' => new Context(ContextDefinition::create('string'), $this->getMode()),
+      'entity' => EntityContext::fromEntity($entity),
+      'display' => EntityContext::fromEntity($this),
+    ] + $this->contextRepository()->getRuntimeContexts($available_context_ids);
+  }
+
+  /**
+   * {@inheritdoc}
+   *
+   * @todo Move this upstream in https://www.drupal.org/node/2939931.
+   */
+  public function label() {
+    $bundle_info = \Drupal::service('entity_type.bundle.info')->getBundleInfo($this->getTargetEntityTypeId());
+    $bundle_label = $bundle_info[$this->getTargetBundle()]['label'];
+    $target_entity_type = $this->entityTypeManager()->getDefinition($this->getTargetEntityTypeId());
+    return new TranslatableMarkup('@bundle @label', ['@bundle' => $bundle_label, '@label' => $target_entity_type->getPluralLabel()]);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function calculateDependencies() {
+    parent::calculateDependencies();
+
+    foreach ($this->getSections() as $section) {
+      $this->calculatePluginDependencies($section->getLayout());
+      foreach ($section->getComponents() as $component) {
+        $this->calculatePluginDependencies($component->getPlugin());
+      }
+    }
+
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function onDependencyRemoval(array $dependencies) {
+    $changed = parent::onDependencyRemoval($dependencies);
+
+    // Loop through all sections and determine if the removed dependencies are
+    // used by their layout plugins.
+    foreach ($this->getSections() as $delta => $section) {
+      $layout_dependencies = $this->getPluginDependencies($section->getLayout());
+      $layout_removed_dependencies = $this->getPluginRemovedDependencies($layout_dependencies, $dependencies);
+      if ($layout_removed_dependencies) {
+        // @todo Allow the plugins to react to their dependency removal in
+        //   https://www.drupal.org/project/drupal/issues/2579743.
+        $this->removeSection($delta);
+        $changed = TRUE;
+      }
+      // If the section is not removed, loop through all components.
+      else {
+        foreach ($section->getComponents() as $uuid => $component) {
+          $plugin_dependencies = $this->getPluginDependencies($component->getPlugin());
+          $component_removed_dependencies = $this->getPluginRemovedDependencies($plugin_dependencies, $dependencies);
+          if ($component_removed_dependencies) {
+            // @todo Allow the plugins to react to their dependency removal in
+            //   https://www.drupal.org/project/drupal/issues/2579743.
+            $section->removeComponent($uuid);
+            $changed = TRUE;
+          }
+        }
+      }
+    }
+    return $changed;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setComponent($name, array $options = []) {
+    parent::setComponent($name, $options);
+
+    // Only continue if Layout Builder is enabled.
+    if (!$this->isLayoutBuilderEnabled()) {
+      return $this;
+    }
+
+    // Retrieve the updated options after the parent:: call.
+    $options = $this->content[$name];
+    // Provide backwards compatibility by converting to a section component.
+    $field_definition = $this->getFieldDefinition($name);
+    $extra_fields = $this->entityFieldManager->getExtraFields($this->getTargetEntityTypeId(), $this->getTargetBundle());
+    $is_view_configurable_non_extra_field = $field_definition && $field_definition->isDisplayConfigurable('view') && isset($options['type']);
+    if ($is_view_configurable_non_extra_field || isset($extra_fields['display'][$name])) {
+      $configuration = [
+        'label_display' => '0',
+        'context_mapping' => ['entity' => 'layout_builder.entity'],
+      ];
+      if ($is_view_configurable_non_extra_field) {
+        $configuration['id'] = 'field_block:' . $this->getTargetEntityTypeId() . ':' . $this->getTargetBundle() . ':' . $name;
+        $keys = array_flip(['type', 'label', 'settings', 'third_party_settings']);
+        $configuration['formatter'] = array_intersect_key($options, $keys);
+      }
+      else {
+        $configuration['id'] = 'extra_field_block:' . $this->getTargetEntityTypeId() . ':' . $this->getTargetBundle() . ':' . $name;
+      }
+
+      $section = $this->getDefaultSection();
+      $region = $options['region'] ?? $section->getDefaultRegion();
+      $new_component = (new SectionComponent(\Drupal::service('uuid')->generate(), $region, $configuration));
+      $section->appendComponent($new_component);
+    }
+    return $this;
+  }
+
+  /**
+   * Gets a default section.
+   *
+   * @return \Drupal\layout_builder\Section
+   *   The default section.
+   */
+  protected function getDefaultSection() {
+    // If no section exists, append a new one.
+    if (!$this->hasSection(0)) {
+      $this->appendSection(new Section('layout_onecol'));
+    }
+
+    // Return the first section.
+    return $this->getSection(0);
+  }
+
+  /**
+   * Gets the section storage manager.
+   *
+   * @return \Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface
+   *   The section storage manager.
+   */
+  private function sectionStorageManager() {
+    return \Drupal::service('plugin.manager.layout_builder.section_storage');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getComponent($name) {
+    if ($this->isLayoutBuilderEnabled() && $section_component = $this->getSectionComponentForFieldName($name)) {
+      $plugin = $section_component->getPlugin();
+      if ($plugin instanceof ConfigurableInterface) {
+        $configuration = $plugin->getConfiguration();
+        if (isset($configuration['formatter'])) {
+          return $configuration['formatter'];
+        }
+      }
+    }
+    return parent::getComponent($name);
+  }
+
+  /**
+   * Gets the component for a given field name if any.
+   *
+   * @param string $field_name
+   *   The field name.
+   *
+   * @return \Drupal\layout_builder\SectionComponent|null
+   *   The section component if it is available.
+   */
+  private function getSectionComponentForFieldName($field_name) {
+    // Loop through every component until the first match is found.
+    foreach ($this->getSections() as $section) {
+      foreach ($section->getComponents() as $component) {
+        $plugin = $component->getPlugin();
+        if ($plugin instanceof DerivativeInspectionInterface && in_array($plugin->getBaseId(), ['field_block', 'extra_field_block'], TRUE)) {
+          // FieldBlock derivative IDs are in the format
+          // [entity_type]:[bundle]:[field].
+          [, , $field_block_field_name] = explode(PluginBase::DERIVATIVE_SEPARATOR, $plugin->getDerivativeId());
+          if ($field_block_field_name === $field_name) {
+            return $component;
+          }
+        }
+      }
+    }
+    return NULL;
+  }
+
+}

core/modules/layout_builder/src/Entity/LayoutBuilderEntityViewDisplayStorage.php

@@ -0,0 +1,44 @@
+<?php
+
+namespace Drupal\layout_builder\Entity;
+
+use Drupal\Core\Config\Entity\ConfigEntityStorage;
+use Drupal\Core\Entity\EntityInterface;
+use Drupal\layout_builder\Section;
+
+/**
+ * Provides storage for entity view display entities that have layouts.
+ *
+ * @internal
+ *   Entity handlers are internal.
+ */
+class LayoutBuilderEntityViewDisplayStorage extends ConfigEntityStorage {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function mapToStorageRecord(EntityInterface $entity) {
+    $record = parent::mapToStorageRecord($entity);
+
+    if (!empty($record['third_party_settings']['layout_builder']['sections'])) {
+      $record['third_party_settings']['layout_builder']['sections'] = array_map(function (Section $section) {
+        return $section->toArray();
+      }, $record['third_party_settings']['layout_builder']['sections']);
+    }
+    return $record;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function mapFromStorageRecords(array $records) {
+    foreach ($records as &$record) {
+      if (!empty($record['third_party_settings']['layout_builder']['sections'])) {
+        $sections = &$record['third_party_settings']['layout_builder']['sections'];
+        $sections = array_map([Section::class, 'fromArray'], $sections);
+      }
+    }
+    return parent::mapFromStorageRecords($records);
+  }
+
+}

core/modules/layout_builder/src/Entity/LayoutBuilderSampleEntityGenerator.php

@@ -0,0 +1,71 @@
+<?php
+
+namespace Drupal\layout_builder\Entity;
+
+use Drupal\Core\Entity\ContentEntityStorageInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\TempStore\SharedTempStoreFactory;
+
+/**
+ * Generates a sample entity for use by the Layout Builder.
+ */
+class LayoutBuilderSampleEntityGenerator implements SampleEntityGeneratorInterface {
+
+  /**
+   * The shared tempstore factory.
+   *
+   * @var \Drupal\Core\TempStore\SharedTempStoreFactory
+   */
+  protected $tempStoreFactory;
+
+  /**
+   * The entity type manager.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
+   */
+  protected $entityTypeManager;
+
+  /**
+   * LayoutBuilderSampleEntityGenerator constructor.
+   *
+   * @param \Drupal\Core\TempStore\SharedTempStoreFactory $temp_store_factory
+   *   The tempstore factory.
+   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
+   *   The entity type manager.
+   */
+  public function __construct(SharedTempStoreFactory $temp_store_factory, EntityTypeManagerInterface $entity_type_manager) {
+    $this->tempStoreFactory = $temp_store_factory;
+    $this->entityTypeManager = $entity_type_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function get($entity_type_id, $bundle_id) {
+    $tempstore = $this->tempStoreFactory->get('layout_builder.sample_entity');
+    if ($entity = $tempstore->get("$entity_type_id.$bundle_id")) {
+      return $entity;
+    }
+
+    $entity_storage = $this->entityTypeManager->getStorage($entity_type_id);
+    if (!$entity_storage instanceof ContentEntityStorageInterface) {
+      throw new \InvalidArgumentException(sprintf('The "%s" entity storage is not supported', $entity_type_id));
+    }
+
+    $entity = $entity_storage->createWithSampleValues($bundle_id);
+    // Mark the sample entity as being a preview.
+    $entity->in_preview = TRUE;
+    $tempstore->set("$entity_type_id.$bundle_id", $entity);
+    return $entity;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function delete($entity_type_id, $bundle_id) {
+    $tempstore = $this->tempStoreFactory->get('layout_builder.sample_entity');
+    $tempstore->delete("$entity_type_id.$bundle_id");
+    return $this;
+  }
+
+}

core/modules/layout_builder/src/Entity/LayoutEntityDisplayInterface.php

@@ -0,0 +1,13 @@
+<?php
+
+namespace Drupal\layout_builder\Entity;
+
+use Drupal\Core\Entity\Display\EntityDisplayInterface;
+use Drupal\layout_builder\LayoutBuilderEnabledInterface;
+use Drupal\layout_builder\SectionListInterface;
+use Drupal\layout_builder\LayoutBuilderOverridableInterface;
+
+/**
+ * Provides an interface for entity displays that have layout.
+ */
+interface LayoutEntityDisplayInterface extends EntityDisplayInterface, SectionListInterface, LayoutBuilderEnabledInterface, LayoutBuilderOverridableInterface {}

core/modules/layout_builder/src/Entity/SampleEntityGeneratorInterface.php

@@ -0,0 +1,35 @@
+<?php
+
+namespace Drupal\layout_builder\Entity;
+
+/**
+ * Generates a sample entity.
+ */
+interface SampleEntityGeneratorInterface {
+
+  /**
+   * Gets a sample entity for a given entity type and bundle.
+   *
+   * @param string $entity_type_id
+   *   The entity type ID.
+   * @param string $bundle_id
+   *   The bundle ID.
+   *
+   * @return \Drupal\Core\Entity\EntityInterface
+   *   An entity.
+   */
+  public function get($entity_type_id, $bundle_id);
+
+  /**
+   * Deletes a sample entity for a given entity type and bundle.
+   *
+   * @param string $entity_type_id
+   *   The entity type ID.
+   * @param string $bundle_id
+   *   The bundle ID.
+   *
+   * @return $this
+   */
+  public function delete($entity_type_id, $bundle_id);
+
+}

core/modules/layout_builder/src/Event/PrepareLayoutEvent.php

@@ -0,0 +1,45 @@
+<?php
+
+namespace Drupal\layout_builder\Event;
+
+use Drupal\layout_builder\SectionStorageInterface;
+use Drupal\Component\EventDispatcher\Event;
+
+/**
+ * Event fired in #pre_render of \Drupal\layout_builder\Element\LayoutBuilder.
+ *
+ * Subscribers to this event can prepare section storage before rendering.
+ *
+ * @see \Drupal\layout_builder\LayoutBuilderEvents::PREPARE_LAYOUT
+ * @see \Drupal\layout_builder\Element\LayoutBuilder::prepareLayout()
+ */
+class PrepareLayoutEvent extends Event {
+
+  /**
+   * The section storage plugin.
+   *
+   * @var \Drupal\layout_builder\SectionStorageInterface
+   */
+  protected $sectionStorage;
+
+  /**
+   * Constructs a new PrepareLayoutEvent.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage preparing the Layout.
+   */
+  public function __construct(SectionStorageInterface $section_storage) {
+    $this->sectionStorage = $section_storage;
+  }
+
+  /**
+   * Gets the section storage.
+   *
+   * @return \Drupal\layout_builder\SectionStorageInterface
+   *   The section storage.
+   */
+  public function getSectionStorage(): SectionStorageInterface {
+    return $this->sectionStorage;
+  }
+
+}

core/modules/layout_builder/src/Event/SectionComponentBuildRenderArrayEvent.php

@@ -0,0 +1,138 @@
+<?php
+
+namespace Drupal\layout_builder\Event;
+
+use Drupal\Core\Cache\CacheableResponseTrait;
+use Drupal\Core\Plugin\PreviewAwarePluginInterface;
+use Drupal\layout_builder\SectionComponent;
+use Drupal\Component\EventDispatcher\Event;
+
+/**
+ * Event fired when a section component's render array is being built.
+ *
+ * Subscribers to this event should manipulate the cacheability object and the
+ * build array in this event.
+ *
+ * @see \Drupal\layout_builder\LayoutBuilderEvents::SECTION_COMPONENT_BUILD_RENDER_ARRAY
+ */
+class SectionComponentBuildRenderArrayEvent extends Event {
+
+  use CacheableResponseTrait;
+
+  /**
+   * The section component whose render array is being built.
+   *
+   * @var \Drupal\layout_builder\SectionComponent
+   */
+  protected $component;
+
+  /**
+   * The available contexts.
+   *
+   * @var \Drupal\Core\Plugin\Context\ContextInterface[]
+   */
+  protected $contexts;
+
+  /**
+   * The plugin for the section component being built.
+   *
+   * @var \Drupal\Component\Plugin\PluginInspectionInterface
+   */
+  protected $plugin;
+
+  /**
+   * Whether the component is in preview mode or not.
+   *
+   * @var bool
+   */
+  protected $inPreview;
+
+  /**
+   * The render array built by the event subscribers.
+   *
+   * @var array
+   */
+  protected $build = [];
+
+  /**
+   * Creates a new SectionComponentBuildRenderArrayEvent object.
+   *
+   * @param \Drupal\layout_builder\SectionComponent $component
+   *   The section component whose render array is being built.
+   * @param \Drupal\Core\Plugin\Context\ContextInterface[] $contexts
+   *   The available contexts.
+   * @param bool $in_preview
+   *   (optional) Whether the component is in preview mode or not.
+   */
+  public function __construct(SectionComponent $component, array $contexts, $in_preview = FALSE) {
+    $this->component = $component;
+    $this->contexts = $contexts;
+    $this->plugin = $component->getPlugin($contexts);
+    $this->inPreview = $in_preview;
+
+    if ($this->plugin instanceof PreviewAwarePluginInterface) {
+      $this->plugin->setInPreview($in_preview);
+    }
+  }
+
+  /**
+   * Get the section component whose render array is being built.
+   *
+   * @return \Drupal\layout_builder\SectionComponent
+   *   The section component whose render array is being built.
+   */
+  public function getComponent() {
+    return $this->component;
+  }
+
+  /**
+   * Get the available contexts.
+   *
+   * @return array|\Drupal\Core\Plugin\Context\ContextInterface[]
+   *   The available contexts.
+   */
+  public function getContexts() {
+    return $this->contexts;
+  }
+
+  /**
+   * Get the plugin for the section component being built.
+   *
+   * @return \Drupal\Component\Plugin\PluginInspectionInterface
+   *   The plugin for the section component being built.
+   */
+  public function getPlugin() {
+    return $this->plugin;
+  }
+
+  /**
+   * Determine if the component is in preview mode.
+   *
+   * @return bool
+   *   Whether the component is in preview mode or not.
+   */
+  public function inPreview() {
+    return $this->inPreview;
+  }
+
+  /**
+   * Get the render array in its current state.
+   *
+   * @return array
+   *   The render array built by the event subscribers.
+   */
+  public function getBuild() {
+    return $this->build;
+  }
+
+  /**
+   * Set the render array.
+   *
+   * @param array $build
+   *   A render array.
+   */
+  public function setBuild(array $build) {
+    $this->build = $build;
+  }
+
+}

core/modules/layout_builder/src/EventSubscriber/BlockComponentRenderArray.php

@@ -0,0 +1,175 @@
+<?php
+
+namespace Drupal\layout_builder\EventSubscriber;
+
+use Drupal\block_content\Access\RefinableDependentAccessInterface;
+use Drupal\Core\Access\AccessResult;
+use Drupal\Core\Block\BlockPluginInterface;
+use Drupal\Core\Cache\CacheableMetadata;
+use Drupal\Core\Render\Element;
+use Drupal\Core\Render\PreviewFallbackInterface;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\layout_builder\Access\LayoutPreviewAccessAllowed;
+use Drupal\layout_builder\Event\SectionComponentBuildRenderArrayEvent;
+use Drupal\layout_builder\Plugin\Block\InlineBlock;
+use Drupal\layout_builder\LayoutBuilderEvents;
+use Drupal\views\Plugin\Block\ViewsBlock;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+/**
+ * Builds render arrays and handles access for all block components.
+ *
+ * @internal
+ *   Tagged services are internal.
+ */
+class BlockComponentRenderArray implements EventSubscriberInterface {
+
+  use StringTranslationTrait;
+
+  /**
+   * The current user.
+   *
+   * @var \Drupal\Core\Session\AccountInterface
+   */
+  protected $currentUser;
+
+  /**
+   * Creates a BlockComponentRenderArray object.
+   *
+   * @param \Drupal\Core\Session\AccountInterface $current_user
+   *   The current user.
+   */
+  public function __construct(AccountInterface $current_user) {
+    $this->currentUser = $current_user;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents(): array {
+    $events[LayoutBuilderEvents::SECTION_COMPONENT_BUILD_RENDER_ARRAY] = ['onBuildRender', 100];
+    return $events;
+  }
+
+  /**
+   * Builds render arrays for block plugins and sets it on the event.
+   *
+   * @param \Drupal\layout_builder\Event\SectionComponentBuildRenderArrayEvent $event
+   *   The section component render event.
+   */
+  public function onBuildRender(SectionComponentBuildRenderArrayEvent $event) {
+    $block = $event->getPlugin();
+    if (!$block instanceof BlockPluginInterface) {
+      return;
+    }
+
+    // Set block access dependency even if we are not checking access on
+    // this level. The block itself may render another
+    // RefinableDependentAccessInterface object and need to pass on this value.
+    if ($block instanceof RefinableDependentAccessInterface) {
+      $contexts = $event->getContexts();
+      if (isset($contexts['layout_builder.entity'])) {
+        if ($entity = $contexts['layout_builder.entity']->getContextValue()) {
+          if ($event->inPreview()) {
+            // If previewing in Layout Builder allow access.
+            $block->setAccessDependency(new LayoutPreviewAccessAllowed());
+          }
+          else {
+            $block->setAccessDependency($entity);
+          }
+        }
+      }
+    }
+
+    // Only check access if the component is not being previewed.
+    if ($event->inPreview()) {
+      $access = AccessResult::allowed()->setCacheMaxAge(0);
+    }
+    else {
+      $access = $block->access($this->currentUser, TRUE);
+    }
+
+    $event->addCacheableDependency($access);
+    if ($access->isAllowed()) {
+      $event->addCacheableDependency($block);
+
+      // @todo Revisit after https://www.drupal.org/node/3027653, as this will
+      //   provide a better way to remove contextual links from Views blocks.
+      //   Currently, doing this requires setting
+      //   \Drupal\views\ViewExecutable::$showAdminLinks() to false before the
+      //   Views block is built.
+      if ($block instanceof ViewsBlock && $event->inPreview()) {
+        $block->getViewExecutable()->setShowAdminLinks(FALSE);
+      }
+
+      $content = $block->build();
+
+      // We don't output the block render data if there are no render elements
+      // found, but we want to capture the cache metadata from the block
+      // regardless.
+      $event->addCacheableDependency(CacheableMetadata::createFromRenderArray($content));
+
+      $is_content_empty = Element::isEmpty($content);
+      $is_placeholder_ready = $event->inPreview() && $block instanceof PreviewFallbackInterface;
+      // If the content is empty and no placeholder is available, return.
+      if ($is_content_empty && !$is_placeholder_ready) {
+        return;
+      }
+
+      $build = [
+        // @todo Move this to BlockBase in https://www.drupal.org/node/2931040.
+        '#theme' => 'block',
+        '#configuration' => $block->getConfiguration(),
+        '#plugin_id' => $block->getPluginId(),
+        '#base_plugin_id' => $block->getBaseId(),
+        '#derivative_plugin_id' => $block->getDerivativeId(),
+        '#in_preview' => $event->inPreview(),
+        '#weight' => $event->getComponent()->getWeight(),
+      ];
+
+      // Place the $content returned by the block plugin into a 'content' child
+      // element, as a way to allow the plugin to have complete control of its
+      // properties and rendering (for instance, its own #theme) without
+      // conflicting with the properties used above, or alternate ones used by
+      // alternate block rendering approaches in contributed modules. However,
+      // the use of a child element is an implementation detail of this
+      // particular block rendering approach. Semantically, the content returned
+      // by the block plugin, and in particular, attributes and contextual links
+      // are information that belong to the entire block. Therefore, we must
+      // move these properties from $content and merge them into the top-level
+      // element.
+      if (isset($content['#attributes'])) {
+        $build['#attributes'] = $content['#attributes'];
+        unset($content['#attributes']);
+      }
+      // Hide contextual links for inline blocks until the UX issues surrounding
+      // editing them directly are resolved.
+      // @see https://www.drupal.org/project/drupal/issues/3075308
+      if (!$block instanceof InlineBlock && !empty($content['#contextual_links'])) {
+        $build['#contextual_links'] = $content['#contextual_links'];
+      }
+      $build['content'] = $content;
+
+      if ($event->inPreview()) {
+        if ($block instanceof PreviewFallbackInterface) {
+          $preview_fallback_string = $block->getPreviewFallbackString();
+        }
+        else {
+          $preview_fallback_string = $this->t('"@block" block', ['@block' => $block->label()]);
+        }
+        // @todo Use new label methods so
+        //   data-layout-content-preview-placeholder-label doesn't have to use
+        //   preview fallback in https://www.drupal.org/node/2025649.
+        $build['#attributes']['data-layout-content-preview-placeholder-label'] = $preview_fallback_string;
+
+        if ($is_content_empty && $is_placeholder_ready) {
+          $build['content']['#markup'] = $this->t('Placeholder for the @preview_fallback', ['@preview_fallback' => $block->getPreviewFallbackString()]);
+        }
+      }
+
+      $event->setBuild($build);
+    }
+  }
+
+}

core/modules/layout_builder/src/EventSubscriber/PrepareLayout.php

@@ -0,0 +1,88 @@
+<?php
+
+namespace Drupal\layout_builder\EventSubscriber;
+
+use Drupal\Core\Messenger\MessengerInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\layout_builder\Event\PrepareLayoutEvent;
+use Drupal\layout_builder\LayoutBuilderEvents;
+use Drupal\layout_builder\LayoutTempstoreRepositoryInterface;
+use Drupal\layout_builder\OverridesSectionStorageInterface;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+/**
+ * An event subscriber to prepare section storage.
+ *
+ * Section storage works via the
+ * \Drupal\layout_builder\Event\PrepareLayoutEvent.
+ *
+ * @see \Drupal\layout_builder\Event\PrepareLayoutEvent
+ * @see \Drupal\layout_builder\Element\LayoutBuilder::prepareLayout()
+ */
+class PrepareLayout implements EventSubscriberInterface {
+
+  use StringTranslationTrait;
+
+  /**
+   * The layout tempstore repository.
+   *
+   * @var \Drupal\layout_builder\LayoutTempstoreRepositoryInterface
+   */
+  protected $layoutTempstoreRepository;
+
+  /**
+   * The messenger service.
+   *
+   * @var \Drupal\Core\Messenger\MessengerInterface
+   */
+  protected $messenger;
+
+  /**
+   * Constructs a new PrepareLayout.
+   *
+   * @param \Drupal\layout_builder\LayoutTempstoreRepositoryInterface $layout_tempstore_repository
+   *   The tempstore repository.
+   * @param \Drupal\Core\Messenger\MessengerInterface $messenger
+   *   The messenger service.
+   */
+  public function __construct(LayoutTempstoreRepositoryInterface $layout_tempstore_repository, MessengerInterface $messenger) {
+    $this->layoutTempstoreRepository = $layout_tempstore_repository;
+    $this->messenger = $messenger;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents(): array {
+    $events[LayoutBuilderEvents::PREPARE_LAYOUT][] = ['onPrepareLayout', 10];
+    return $events;
+  }
+
+  /**
+   * Prepares a layout for use in the UI.
+   *
+   * @param \Drupal\layout_builder\Event\PrepareLayoutEvent $event
+   *   The prepare layout event.
+   */
+  public function onPrepareLayout(PrepareLayoutEvent $event) {
+    $section_storage = $event->getSectionStorage();
+
+    // If the layout has pending changes, add a warning.
+    if ($this->layoutTempstoreRepository->has($section_storage)) {
+      $this->messenger->addWarning($this->t('You have unsaved changes.'));
+    }
+    else {
+      // If the layout is an override that has not yet been overridden, copy the
+      // sections from the corresponding default.
+      if ($section_storage instanceof OverridesSectionStorageInterface && !$section_storage->isOverridden()) {
+        $sections = $section_storage->getDefaultSectionStorage()->getSections();
+        foreach ($sections as $section) {
+          $section_storage->appendSection($section);
+        }
+      }
+      // Add storage to tempstore regardless of what the storage is.
+      $this->layoutTempstoreRepository->set($section_storage);
+    }
+  }
+
+}

core/modules/layout_builder/src/EventSubscriber/SetInlineBlockDependency.php

@@ -0,0 +1,160 @@
+<?php
+
+namespace Drupal\layout_builder\EventSubscriber;
+
+use Drupal\block_content\BlockContentEvents;
+use Drupal\block_content\BlockContentInterface;
+use Drupal\block_content\Event\BlockContentGetDependencyEvent;
+use Drupal\Core\Database\Connection;
+use Drupal\Core\Entity\EntityInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\layout_builder\InlineBlockUsageInterface;
+use Drupal\layout_builder\LayoutEntityHelperTrait;
+use Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+/**
+ * An event subscriber that returns an access dependency for inline blocks.
+ *
+ * When used within the layout builder the access dependency for inline blocks
+ * will be explicitly set but if access is evaluated outside of the layout
+ * builder then the dependency may not have been set.
+ *
+ * A known example of when the access dependency will not have been set is when
+ * determining 'view' or 'download' access to a file entity that is attached
+ * to a content block via a field that is using the private file system. The
+ * file access handler will evaluate access on the content block without setting
+ * the dependency.
+ *
+ * @internal
+ *   Tagged services are internal.
+ *
+ * @see \Drupal\file\FileAccessControlHandler::checkAccess()
+ * @see \Drupal\block_content\BlockContentAccessControlHandler::checkAccess()
+ */
+class SetInlineBlockDependency implements EventSubscriberInterface {
+
+  use LayoutEntityHelperTrait;
+
+  /**
+   * The entity type manager.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
+   */
+  protected $entityTypeManager;
+
+  /**
+   * The database connection.
+   *
+   * @var \Drupal\Core\Database\Connection
+   */
+  protected $database;
+
+  /**
+   * The inline block usage service.
+   *
+   * @var \Drupal\layout_builder\InlineBlockUsageInterface
+   */
+  protected $usage;
+
+  /**
+   * Constructs SetInlineBlockDependency object.
+   *
+   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
+   *   The entity type manager.
+   * @param \Drupal\Core\Database\Connection $database
+   *   The database connection.
+   * @param \Drupal\layout_builder\InlineBlockUsageInterface $usage
+   *   The inline block usage service.
+   * @param \Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface $section_storage_manager
+   *   The section storage manager.
+   */
+  public function __construct(EntityTypeManagerInterface $entity_type_manager, Connection $database, InlineBlockUsageInterface $usage, SectionStorageManagerInterface $section_storage_manager) {
+    $this->entityTypeManager = $entity_type_manager;
+    $this->database = $database;
+    $this->usage = $usage;
+    $this->sectionStorageManager = $section_storage_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents(): array {
+    return [
+      BlockContentEvents::BLOCK_CONTENT_GET_DEPENDENCY => 'onGetDependency',
+    ];
+  }
+
+  /**
+   * Handles the BlockContentEvents::INLINE_BLOCK_GET_DEPENDENCY event.
+   *
+   * @param \Drupal\block_content\Event\BlockContentGetDependencyEvent $event
+   *   The event.
+   */
+  public function onGetDependency(BlockContentGetDependencyEvent $event) {
+    if ($dependency = $this->getInlineBlockDependency($event->getBlockContentEntity())) {
+      $event->setAccessDependency($dependency);
+    }
+  }
+
+  /**
+   * Get the access dependency of an inline block.
+   *
+   * If the block is used in an entity that entity will be returned as the
+   * dependency.
+   *
+   * For revisionable entities the entity will only be returned if it is used in
+   * the latest revision of the entity. For inline blocks that are not used in
+   * the latest revision but are used in a previous revision the entity will not
+   * be returned because calling
+   * \Drupal\Core\Access\AccessibleInterface::access() will only check access on
+   * the latest revision. Therefore if the previous revision of the entity was
+   * returned as the dependency access would be granted to inline block
+   * regardless of whether the user has access to the revision in which the
+   * inline block was used.
+   *
+   * @param \Drupal\block_content\BlockContentInterface $block_content
+   *   The block content entity.
+   *
+   * @return \Drupal\Core\Entity\EntityInterface|null
+   *   Returns the layout dependency.
+   *
+   * @see \Drupal\block_content\BlockContentAccessControlHandler::checkAccess()
+   * @see \Drupal\layout_builder\EventSubscriber\BlockComponentRenderArray::onBuildRender()
+   */
+  protected function getInlineBlockDependency(BlockContentInterface $block_content) {
+    $layout_entity_info = $this->usage->getUsage($block_content->id());
+    if (empty($layout_entity_info)) {
+      // If the block does not have usage information then we cannot set a
+      // dependency. It may be used by another module besides layout builder.
+      return NULL;
+    }
+    $layout_entity_storage = $this->entityTypeManager->getStorage($layout_entity_info->layout_entity_type);
+    $layout_entity = $layout_entity_storage->load($layout_entity_info->layout_entity_id);
+    if ($this->isLayoutCompatibleEntity($layout_entity)) {
+      if ($this->isBlockRevisionUsedInEntity($layout_entity, $block_content)) {
+        return $layout_entity;
+      }
+
+    }
+    return NULL;
+  }
+
+  /**
+   * Determines if a block content revision is used in an entity.
+   *
+   * @param \Drupal\Core\Entity\EntityInterface $layout_entity
+   *   The layout entity.
+   * @param \Drupal\block_content\BlockContentInterface $block_content
+   *   The block content revision.
+   *
+   * @return bool
+   *   TRUE if the block content revision is used as an inline block in the
+   *   layout entity.
+   */
+  protected function isBlockRevisionUsedInEntity(EntityInterface $layout_entity, BlockContentInterface $block_content) {
+    $sections_blocks_revision_ids = $this->getInlineBlockRevisionIdsInSections($this->getEntitySections($layout_entity));
+    return in_array($block_content->getRevisionId(), $sections_blocks_revision_ids);
+  }
+
+}

core/modules/layout_builder/src/Field/LayoutSectionItemList.php

@@ -0,0 +1,108 @@
+<?php
+
+namespace Drupal\layout_builder\Field;
+
+use Drupal\Core\Access\AccessResult;
+use Drupal\Core\Field\FieldItemList;
+use Drupal\Core\Field\FieldItemListInterface;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\layout_builder\Section;
+use Drupal\layout_builder\SectionListInterface;
+use Drupal\layout_builder\SectionListTrait;
+
+/**
+ * Defines an item list class for layout section fields.
+ *
+ * @internal
+ *   Plugin classes are internal.
+ *
+ * @see \Drupal\layout_builder\Plugin\Field\FieldType\LayoutSectionItem
+ */
+class LayoutSectionItemList extends FieldItemList implements SectionListInterface {
+
+  use SectionListTrait;
+
+  /**
+   * Numerically indexed array of field items.
+   *
+   * @var \Drupal\layout_builder\Plugin\Field\FieldType\LayoutSectionItem[]
+   */
+  protected $list = [];
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getSections() {
+    $sections = [];
+    foreach ($this->list as $delta => $item) {
+      $sections[$delta] = $item->section;
+    }
+    return $sections;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setSections(array $sections) {
+    $this->list = [];
+    $sections = array_values($sections);
+    /** @var \Drupal\layout_builder\Plugin\Field\FieldType\LayoutSectionItem $item */
+    foreach ($sections as $section) {
+      $item = $this->appendItem();
+      $item->section = $section;
+    }
+
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getEntity() {
+    $entity = parent::getEntity();
+
+    // Ensure the entity is updated with the latest value.
+    $entity->set($this->getName(), $this->getValue());
+    return $entity;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function preSave() {
+    parent::preSave();
+    // Loop through each section and reconstruct it to ensure that all default
+    // values are present.
+    foreach ($this->list as $item) {
+      $item->section = Section::fromArray($item->section->toArray());
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function equals(FieldItemListInterface $list_to_compare) {
+    if (!$list_to_compare instanceof LayoutSectionItemList) {
+      return FALSE;
+    }
+
+    // Convert arrays of section objects to array values for comparison.
+    $convert = function (LayoutSectionItemList $list) {
+      return array_map(function (Section $section) {
+        return $section->toArray();
+      }, $list->getSections());
+    };
+    return $convert($this) === $convert($list_to_compare);
+  }
+
+  /**
+   * Overrides \Drupal\Core\Field\FieldItemListInterface::defaultAccess().
+   *
+   * @ingroup layout_builder_access
+   */
+  public function defaultAccess($operation = 'view', ?AccountInterface $account = NULL) {
+    // @todo Allow access in https://www.drupal.org/node/2942975.
+    return AccessResult::forbidden();
+  }
+
+}

core/modules/layout_builder/src/Form/AddBlockForm.php

@@ -0,0 +1,64 @@
+<?php
+
+namespace Drupal\layout_builder\Form;
+
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\layout_builder\LayoutBuilderHighlightTrait;
+use Drupal\layout_builder\SectionComponent;
+use Drupal\layout_builder\SectionStorageInterface;
+
+/**
+ * Provides a form to add a block.
+ *
+ * @internal
+ *   Form classes are internal.
+ */
+class AddBlockForm extends ConfigureBlockFormBase {
+
+  use LayoutBuilderHighlightTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'layout_builder_add_block';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function submitLabel() {
+    return $this->t('Add block');
+  }
+
+  /**
+   * Builds the form for the block.
+   *
+   * @param array $form
+   *   An associative array containing the structure of the form.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage being configured.
+   * @param int $delta
+   *   The delta of the section.
+   * @param string $region
+   *   The region of the block.
+   * @param string|null $plugin_id
+   *   The plugin ID of the block to add.
+   *
+   * @return array
+   *   The form array.
+   */
+  public function buildForm(array $form, FormStateInterface $form_state, ?SectionStorageInterface $section_storage = NULL, $delta = NULL, $region = NULL, $plugin_id = NULL) {
+    // Only generate a new component once per form submission.
+    if (!$component = $form_state->get('layout_builder__component')) {
+      $component = new SectionComponent($this->uuidGenerator->generate(), $region, ['id' => $plugin_id]);
+      $section_storage->getSection($delta)->appendComponent($component);
+      $form_state->set('layout_builder__component', $component);
+    }
+    $form['#attributes']['data-layout-builder-target-highlight-id'] = $this->blockAddHighlightId($delta, $region);
+    return $this->doBuildForm($form, $form_state, $section_storage, $delta, $component);
+  }
+
+}

core/modules/layout_builder/src/Form/ConfigureBlockFormBase.php

@@ -0,0 +1,290 @@
+<?php
+
+namespace Drupal\layout_builder\Form;
+
+use Drupal\Component\Utility\Html;
+use Drupal\Component\Uuid\UuidInterface;
+use Drupal\Core\Ajax\AjaxFormHelperTrait;
+use Drupal\Core\Block\BlockManagerInterface;
+use Drupal\Core\Block\BlockPluginInterface;
+use Drupal\Core\Form\BaseFormIdInterface;
+use Drupal\Core\Form\FormBase;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Form\SubformState;
+use Drupal\Core\Form\WorkspaceDynamicSafeFormInterface;
+use Drupal\Core\Plugin\Context\ContextRepositoryInterface;
+use Drupal\Core\Plugin\ContextAwarePluginAssignmentTrait;
+use Drupal\Core\Plugin\ContextAwarePluginInterface;
+use Drupal\Core\Plugin\PluginFormFactoryInterface;
+use Drupal\Core\Plugin\PluginWithFormsInterface;
+use Drupal\layout_builder\Context\LayoutBuilderContextTrait;
+use Drupal\layout_builder\Controller\LayoutRebuildTrait;
+use Drupal\layout_builder\LayoutTempstoreRepositoryInterface;
+use Drupal\layout_builder\SectionComponent;
+use Drupal\layout_builder\SectionStorageInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Provides a base form for configuring a block.
+ *
+ * @internal
+ *   Form classes are internal.
+ */
+abstract class ConfigureBlockFormBase extends FormBase implements BaseFormIdInterface, WorkspaceDynamicSafeFormInterface {
+
+  use AjaxFormHelperTrait;
+  use ContextAwarePluginAssignmentTrait;
+  use LayoutBuilderContextTrait;
+  use LayoutRebuildTrait;
+  use WorkspaceSafeFormTrait;
+
+  /**
+   * The plugin being configured.
+   *
+   * @var \Drupal\Core\Block\BlockPluginInterface
+   */
+  protected $block;
+
+  /**
+   * The layout tempstore repository.
+   *
+   * @var \Drupal\layout_builder\LayoutTempstoreRepositoryInterface
+   */
+  protected $layoutTempstoreRepository;
+
+  /**
+   * The block manager.
+   *
+   * @var \Drupal\Core\Block\BlockManagerInterface
+   */
+  protected $blockManager;
+
+  /**
+   * The UUID generator.
+   *
+   * @var \Drupal\Component\Uuid\UuidInterface
+   */
+  protected $uuidGenerator;
+
+  /**
+   * The plugin form manager.
+   *
+   * @var \Drupal\Core\Plugin\PluginFormFactoryInterface
+   */
+  protected $pluginFormFactory;
+
+  /**
+   * The field delta.
+   *
+   * @var int
+   */
+  protected $delta;
+
+  /**
+   * The current region.
+   *
+   * @var string
+   */
+  protected $region;
+
+  /**
+   * The UUID of the component.
+   *
+   * @var string
+   */
+  protected $uuid;
+
+  /**
+   * The section storage.
+   *
+   * @var \Drupal\layout_builder\SectionStorageInterface
+   */
+  protected $sectionStorage;
+
+  /**
+   * Constructs a new block form.
+   *
+   * @param \Drupal\layout_builder\LayoutTempstoreRepositoryInterface $layout_tempstore_repository
+   *   The layout tempstore repository.
+   * @param \Drupal\Core\Plugin\Context\ContextRepositoryInterface $context_repository
+   *   The context repository.
+   * @param \Drupal\Core\Block\BlockManagerInterface $block_manager
+   *   The block manager.
+   * @param \Drupal\Component\Uuid\UuidInterface $uuid
+   *   The UUID generator.
+   * @param \Drupal\Core\Plugin\PluginFormFactoryInterface $plugin_form_manager
+   *   The plugin form manager.
+   */
+  public function __construct(LayoutTempstoreRepositoryInterface $layout_tempstore_repository, ContextRepositoryInterface $context_repository, BlockManagerInterface $block_manager, UuidInterface $uuid, PluginFormFactoryInterface $plugin_form_manager) {
+    $this->layoutTempstoreRepository = $layout_tempstore_repository;
+    $this->contextRepository = $context_repository;
+    $this->blockManager = $block_manager;
+    $this->uuidGenerator = $uuid;
+    $this->pluginFormFactory = $plugin_form_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('layout_builder.tempstore_repository'),
+      $container->get('context.repository'),
+      $container->get('plugin.manager.block'),
+      $container->get('uuid'),
+      $container->get('plugin_form.factory')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getBaseFormId() {
+    return 'layout_builder_configure_block';
+  }
+
+  /**
+   * Builds the form for the block.
+   *
+   * @param array $form
+   *   An associative array containing the structure of the form.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage being configured.
+   * @param int $delta
+   *   The delta of the section.
+   * @param \Drupal\layout_builder\SectionComponent $component
+   *   The section component containing the block.
+   *
+   * @return array
+   *   The form array.
+   */
+  public function doBuildForm(array $form, FormStateInterface $form_state, ?SectionStorageInterface $section_storage = NULL, $delta = NULL, ?SectionComponent $component = NULL) {
+    $this->sectionStorage = $section_storage;
+    $this->delta = $delta;
+    $this->uuid = $component->getUuid();
+    $this->block = $component->getPlugin();
+
+    $form_state->setTemporaryValue('gathered_contexts', $this->getPopulatedContexts($section_storage));
+
+    $form['#tree'] = TRUE;
+    $form['settings'] = [];
+    $subform_state = SubformState::createForSubform($form['settings'], $form, $form_state);
+    $form['settings'] = $this->getPluginForm($this->block)->buildConfigurationForm($form['settings'], $subform_state);
+
+    $form['actions']['submit'] = [
+      '#type' => 'submit',
+      '#value' => $this->submitLabel(),
+      '#button_type' => 'primary',
+    ];
+    if ($this->isAjax()) {
+      $form['actions']['submit']['#ajax']['callback'] = '::ajaxSubmit';
+      // @todo static::ajaxSubmit() requires data-drupal-selector to be the same
+      //   between the various Ajax requests. A bug in
+      //   \Drupal\Core\Form\FormBuilder prevents that from happening unless
+      //   $form['#id'] is also the same. Normally, #id is set to a unique HTML
+      //   ID via Html::getUniqueId(), but here we bypass that in order to work
+      //   around the data-drupal-selector bug. This is okay so long as we
+      //   assume that this form only ever occurs once on a page. Remove this
+      //   workaround in https://www.drupal.org/node/2897377.
+      $form['#id'] = Html::getId($form_state->getBuildInfo()['form_id']);
+    }
+
+    // Mark this as an administrative page for JavaScript ("Back to site" link).
+    $form['#attached']['drupalSettings']['path']['currentPathIsAdmin'] = TRUE;
+    return $form;
+  }
+
+  /**
+   * Returns the label for the submit button.
+   *
+   * @return string
+   *   Submit label.
+   */
+  abstract protected function submitLabel();
+
+  /**
+   * {@inheritdoc}
+   */
+  public function validateForm(array &$form, FormStateInterface $form_state) {
+    $subform_state = SubformState::createForSubform($form['settings'], $form, $form_state);
+    $this->getPluginForm($this->block)->validateConfigurationForm($form['settings'], $subform_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+    // Call the plugin submit handler.
+    $subform_state = SubformState::createForSubform($form['settings'], $form, $form_state);
+    $this->getPluginForm($this->block)->submitConfigurationForm($form, $subform_state);
+
+    // If this block is context-aware, set the context mapping.
+    if ($this->block instanceof ContextAwarePluginInterface) {
+      $this->block->setContextMapping($subform_state->getValue('context_mapping', []));
+    }
+
+    $configuration = $this->block->getConfiguration();
+
+    $section = $this->sectionStorage->getSection($this->delta);
+    $section->getComponent($this->uuid)->setConfiguration($configuration);
+
+    $this->layoutTempstoreRepository->set($this->sectionStorage);
+    $form_state->setRedirectUrl($this->sectionStorage->getLayoutBuilderUrl());
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function successfulAjaxSubmit(array $form, FormStateInterface $form_state) {
+    return $this->rebuildAndClose($this->sectionStorage);
+  }
+
+  /**
+   * Retrieves the plugin form for a given block.
+   *
+   * @param \Drupal\Core\Block\BlockPluginInterface $block
+   *   The block plugin.
+   *
+   * @return \Drupal\Core\Plugin\PluginFormInterface
+   *   The plugin form for the block.
+   */
+  protected function getPluginForm(BlockPluginInterface $block) {
+    if ($block instanceof PluginWithFormsInterface) {
+      return $this->pluginFormFactory->createInstance($block, 'configure');
+    }
+    return $block;
+  }
+
+  /**
+   * Retrieves the section storage object.
+   *
+   * @return \Drupal\layout_builder\SectionStorageInterface
+   *   The section storage for the current form.
+   */
+  public function getSectionStorage() {
+    return $this->sectionStorage;
+  }
+
+  /**
+   * Retrieves the current layout section being edited by the form.
+   *
+   * @return \Drupal\layout_builder\Section
+   *   The current layout section.
+   */
+  public function getCurrentSection() {
+    return $this->sectionStorage->getSection($this->delta);
+  }
+
+  /**
+   * Retrieves the current component being edited by the form.
+   *
+   * @return \Drupal\layout_builder\SectionComponent
+   *   The current section component.
+   */
+  public function getCurrentComponent() {
+    return $this->getCurrentSection()->getComponent($this->uuid);
+  }
+
+}

core/modules/layout_builder/src/Form/ConfigureSectionForm.php

@@ -0,0 +1,278 @@
+<?php
+
+namespace Drupal\layout_builder\Form;
+
+use Drupal\Component\Utility\Html;
+use Drupal\Core\Ajax\AjaxFormHelperTrait;
+use Drupal\Core\Form\FormBase;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Form\SubformState;
+use Drupal\Core\Form\WorkspaceDynamicSafeFormInterface;
+use Drupal\Core\Layout\LayoutInterface;
+use Drupal\Core\Plugin\ContextAwarePluginInterface;
+use Drupal\Core\Plugin\PluginFormFactoryInterface;
+use Drupal\Core\Plugin\PluginFormInterface;
+use Drupal\Core\Plugin\PluginWithFormsInterface;
+use Drupal\layout_builder\Context\LayoutBuilderContextTrait;
+use Drupal\layout_builder\Controller\LayoutRebuildTrait;
+use Drupal\layout_builder\LayoutBuilderHighlightTrait;
+use Drupal\layout_builder\LayoutTempstoreRepositoryInterface;
+use Drupal\layout_builder\Section;
+use Drupal\layout_builder\SectionStorageInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Provides a form for configuring a layout section.
+ *
+ * @internal
+ *   Form classes are internal.
+ */
+class ConfigureSectionForm extends FormBase implements WorkspaceDynamicSafeFormInterface {
+
+  use AjaxFormHelperTrait;
+  use LayoutBuilderContextTrait;
+  use LayoutBuilderHighlightTrait;
+  use LayoutRebuildTrait;
+  use WorkspaceSafeFormTrait;
+
+  /**
+   * The layout tempstore repository.
+   *
+   * @var \Drupal\layout_builder\LayoutTempstoreRepositoryInterface
+   */
+  protected $layoutTempstoreRepository;
+
+  /**
+   * The plugin being configured.
+   *
+   * @var \Drupal\Core\Layout\LayoutInterface|\Drupal\Core\Plugin\PluginFormInterface
+   */
+  protected $layout;
+
+  /**
+   * The section being configured.
+   *
+   * @var \Drupal\layout_builder\Section
+   */
+  protected $section;
+
+  /**
+   * The plugin form manager.
+   *
+   * @var \Drupal\Core\Plugin\PluginFormFactoryInterface
+   */
+  protected $pluginFormFactory;
+
+  /**
+   * The section storage.
+   *
+   * @var \Drupal\layout_builder\SectionStorageInterface
+   */
+  protected $sectionStorage;
+
+  /**
+   * The field delta.
+   *
+   * @var int
+   */
+  protected $delta;
+
+  /**
+   * The plugin ID.
+   *
+   * @var string
+   */
+  protected $pluginId;
+
+  /**
+   * Indicates whether the section is being added or updated.
+   *
+   * @var bool
+   */
+  protected $isUpdate;
+
+  /**
+   * Constructs a new ConfigureSectionForm.
+   *
+   * @param \Drupal\layout_builder\LayoutTempstoreRepositoryInterface $layout_tempstore_repository
+   *   The layout tempstore repository.
+   * @param \Drupal\Core\Plugin\PluginFormFactoryInterface $plugin_form_manager
+   *   The plugin form manager.
+   */
+  public function __construct(LayoutTempstoreRepositoryInterface $layout_tempstore_repository, PluginFormFactoryInterface $plugin_form_manager) {
+    $this->layoutTempstoreRepository = $layout_tempstore_repository;
+    $this->pluginFormFactory = $plugin_form_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('layout_builder.tempstore_repository'),
+      $container->get('plugin_form.factory')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'layout_builder_configure_section';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state, ?SectionStorageInterface $section_storage = NULL, $delta = NULL, $plugin_id = NULL) {
+    $this->sectionStorage = $section_storage;
+    $this->delta = $delta;
+    $this->isUpdate = is_null($plugin_id);
+    $this->pluginId = $plugin_id;
+
+    $section = $this->getCurrentSection();
+
+    if ($this->isUpdate) {
+      if ($label = $section->getLayoutSettings()['label']) {
+        $form['#title'] = $this->t('Configure @section', ['@section' => $label]);
+      }
+    }
+    // Passing available contexts to the layout plugin here could result in an
+    // exception since the layout may not have a context mapping for a required
+    // context slot on creation.
+    $this->layout = $section->getLayout();
+
+    $form_state->setTemporaryValue('gathered_contexts', $this->getPopulatedContexts($this->sectionStorage));
+    $form['#tree'] = TRUE;
+    $form['layout_settings'] = [];
+    $subform_state = SubformState::createForSubform($form['layout_settings'], $form, $form_state);
+    $form['layout_settings'] = $this->getPluginForm($this->layout)->buildConfigurationForm($form['layout_settings'], $subform_state);
+
+    $form['actions']['submit'] = [
+      '#type' => 'submit',
+      '#value' => $this->isUpdate ? $this->t('Update') : $this->t('Add section'),
+      '#button_type' => 'primary',
+    ];
+    if ($this->isAjax()) {
+      $form['actions']['submit']['#ajax']['callback'] = '::ajaxSubmit';
+      // @todo static::ajaxSubmit() requires data-drupal-selector to be the same
+      //   between the various Ajax requests. A bug in
+      //   \Drupal\Core\Form\FormBuilder prevents that from happening unless
+      //   $form['#id'] is also the same. Normally, #id is set to a unique HTML
+      //   ID via Html::getUniqueId(), but here we bypass that in order to work
+      //   around the data-drupal-selector bug. This is okay so long as we
+      //   assume that this form only ever occurs once on a page. Remove this
+      //   workaround in https://www.drupal.org/node/2897377.
+      $form['#id'] = Html::getId($form_state->getBuildInfo()['form_id']);
+    }
+    $target_highlight_id = $this->isUpdate ? $this->sectionUpdateHighlightId($delta) : $this->sectionAddHighlightId($delta);
+    $form['#attributes']['data-layout-builder-target-highlight-id'] = $target_highlight_id;
+
+    // Mark this as an administrative page for JavaScript ("Back to site" link).
+    $form['#attached']['drupalSettings']['path']['currentPathIsAdmin'] = TRUE;
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function validateForm(array &$form, FormStateInterface $form_state) {
+    $subform_state = SubformState::createForSubform($form['layout_settings'], $form, $form_state);
+    $this->getPluginForm($this->layout)->validateConfigurationForm($form['layout_settings'], $subform_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+    // Call the plugin submit handler.
+    $subform_state = SubformState::createForSubform($form['layout_settings'], $form, $form_state);
+    $this->getPluginForm($this->layout)->submitConfigurationForm($form['layout_settings'], $subform_state);
+
+    // If this layout is context-aware, set the context mapping.
+    if ($this->layout instanceof ContextAwarePluginInterface) {
+      $this->layout->setContextMapping($subform_state->getValue('context_mapping', []));
+    }
+
+    $configuration = $this->layout->getConfiguration();
+
+    $section = $this->getCurrentSection();
+    $section->setLayoutSettings($configuration);
+    if (!$this->isUpdate) {
+      $this->sectionStorage->insertSection($this->delta, $section);
+    }
+
+    $this->layoutTempstoreRepository->set($this->sectionStorage);
+    $form_state->setRedirectUrl($this->sectionStorage->getLayoutBuilderUrl());
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function successfulAjaxSubmit(array $form, FormStateInterface $form_state) {
+    return $this->rebuildAndClose($this->sectionStorage);
+  }
+
+  /**
+   * Retrieves the plugin form for a given layout.
+   *
+   * @param \Drupal\Core\Layout\LayoutInterface $layout
+   *   The layout plugin.
+   *
+   * @return \Drupal\Core\Plugin\PluginFormInterface
+   *   The plugin form for the layout.
+   *
+   * @throws \Drupal\Component\Plugin\Exception\InvalidPluginDefinitionException
+   */
+  protected function getPluginForm(LayoutInterface $layout) {
+    if ($layout instanceof PluginWithFormsInterface) {
+      return $this->pluginFormFactory->createInstance($layout, 'configure');
+    }
+
+    if ($layout instanceof PluginFormInterface) {
+      return $layout;
+    }
+
+    throw new \InvalidArgumentException(sprintf('The "%s" layout does not provide a configuration form', $layout->getPluginId()));
+  }
+
+  /**
+   * Retrieves the section storage property.
+   *
+   * @return \Drupal\layout_builder\SectionStorageInterface
+   *   The section storage for the current form.
+   */
+  public function getSectionStorage() {
+    return $this->sectionStorage;
+  }
+
+  /**
+   * Retrieves the layout being modified by the form.
+   *
+   * @return \Drupal\Core\Layout\LayoutInterface|\Drupal\Core\Plugin\PluginFormInterface
+   *   The layout for the current form.
+   */
+  public function getCurrentLayout(): LayoutInterface {
+    return $this->layout;
+  }
+
+  /**
+   * Retrieves the section being modified by the form.
+   *
+   * @return \Drupal\layout_builder\Section
+   *   The section for the current form.
+   */
+  public function getCurrentSection(): Section {
+    if (!isset($this->section)) {
+      if ($this->isUpdate) {
+        $this->section = $this->sectionStorage->getSection($this->delta);
+      }
+      else {
+        $this->section = new Section($this->pluginId);
+      }
+    }
+
+    return $this->section;
+  }
+
+}

core/modules/layout_builder/src/Form/DefaultsEntityForm.php

@@ -0,0 +1,164 @@
+<?php
+
+namespace Drupal\layout_builder\Form;
+
+use Drupal\Core\Entity\EntityForm;
+use Drupal\Core\Entity\EntityTypeBundleInfoInterface;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Routing\RouteMatchInterface;
+use Drupal\layout_builder\Entity\LayoutEntityDisplayInterface;
+use Drupal\layout_builder\LayoutTempstoreRepositoryInterface;
+use Drupal\layout_builder\SectionStorageInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Provides a form containing the Layout Builder UI for defaults.
+ *
+ * @internal
+ *   Form classes are internal.
+ */
+class DefaultsEntityForm extends EntityForm {
+
+  use PreviewToggleTrait;
+  use LayoutBuilderEntityFormTrait;
+
+  /**
+   * Layout tempstore repository.
+   *
+   * @var \Drupal\layout_builder\LayoutTempstoreRepositoryInterface
+   */
+  protected $layoutTempstoreRepository;
+
+  /**
+   * The entity type bundle info service.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeBundleInfoInterface
+   */
+  protected $entityTypeBundleInfo;
+
+  /**
+   * The section storage.
+   *
+   * @var \Drupal\layout_builder\SectionStorageInterface
+   */
+  protected $sectionStorage;
+
+  /**
+   * Constructs a new DefaultsEntityForm.
+   *
+   * @param \Drupal\layout_builder\LayoutTempstoreRepositoryInterface $layout_tempstore_repository
+   *   The layout tempstore repository.
+   * @param \Drupal\Core\Entity\EntityTypeBundleInfoInterface $entity_type_bundle_info
+   *   The entity type bundle info service.
+   */
+  public function __construct(LayoutTempstoreRepositoryInterface $layout_tempstore_repository, EntityTypeBundleInfoInterface $entity_type_bundle_info) {
+    $this->layoutTempstoreRepository = $layout_tempstore_repository;
+    $this->entityTypeBundleInfo = $entity_type_bundle_info;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('layout_builder.tempstore_repository'),
+      $container->get('entity_type.bundle.info')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state, ?SectionStorageInterface $section_storage = NULL) {
+    $form['#attributes']['class'][] = 'layout-builder-form';
+    $form['layout_builder'] = [
+      '#type' => 'layout_builder',
+      '#section_storage' => $section_storage,
+      '#process' => [[static::class, 'layoutBuilderElementGetKeys']],
+    ];
+    $form['layout_builder_message'] = $this->buildMessage($section_storage->getContextValue('display'));
+
+    $this->sectionStorage = $section_storage;
+    return parent::buildForm($form, $form_state);
+  }
+
+  /**
+   * Form element #process callback.
+   *
+   * Save the layout builder element array parents as a property on the top form
+   * element so that they can be used to access the element within the whole
+   * render array later.
+   *
+   * @see \Drupal\layout_builder\Controller\LayoutBuilderHtmlEntityFormController
+   */
+  public static function layoutBuilderElementGetKeys(array $element, FormStateInterface $form_state, &$form) {
+    $form['#layout_builder_element_keys'] = $element['#array_parents'];
+    return $element;
+  }
+
+  /**
+   * Renders a message to display at the top of the layout builder.
+   *
+   * @param \Drupal\layout_builder\Entity\LayoutEntityDisplayInterface $entity
+   *   The entity view display being edited.
+   *
+   * @return array
+   *   A renderable array containing the message.
+   */
+  protected function buildMessage(LayoutEntityDisplayInterface $entity) {
+    $entity_type_id = $entity->getTargetEntityTypeId();
+    $entity_type = $this->entityTypeManager->getDefinition($entity_type_id);
+    $bundle_info = $this->entityTypeBundleInfo->getBundleInfo($entity_type_id);
+
+    $args = [
+      '@bundle' => $bundle_info[$entity->getTargetBundle()]['label'],
+      '@plural_label' => $entity_type->getPluralLabel(),
+    ];
+    if ($entity_type->hasKey('bundle')) {
+      $message = $this->t('You are editing the layout template for all @bundle @plural_label.', $args);
+    }
+    else {
+      $message = $this->t('You are editing the layout template for all @plural_label.', $args);
+    }
+    return $this->buildMessageContainer($message, 'defaults');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildEntity(array $form, FormStateInterface $form_state) {
+    // \Drupal\Core\Entity\EntityForm::buildEntity() clones the entity object.
+    // Keep it in sync with the one used by the section storage.
+    $this->setEntity($this->sectionStorage->getContextValue('display'));
+    $entity = parent::buildEntity($form, $form_state);
+    $this->sectionStorage->setContextValue('display', $entity);
+    return $entity;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getEntityFromRouteMatch(RouteMatchInterface $route_match, $entity_type_id) {
+    $route_parameters = $route_match->getParameters()->all();
+
+    return $this->entityTypeManager->getStorage('entity_view_display')->load($route_parameters['entity_type_id'] . '.' . $route_parameters['bundle'] . '.' . $route_parameters['view_mode_name']);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function actions(array $form, FormStateInterface $form_state) {
+    $actions = parent::actions($form, $form_state);
+    return $this->buildActions($actions);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function save(array $form, FormStateInterface $form_state) {
+    $return = $this->sectionStorage->save();
+    $this->saveTasks($form_state, $this->t('The layout has been saved.'));
+    return $return;
+  }
+
+}

core/modules/layout_builder/src/Form/DiscardLayoutChangesForm.php

@@ -0,0 +1,109 @@
+<?php
+
+namespace Drupal\layout_builder\Form;
+
+use Drupal\Core\Form\ConfirmFormBase;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Form\WorkspaceDynamicSafeFormInterface;
+use Drupal\Core\Messenger\MessengerInterface;
+use Drupal\layout_builder\LayoutTempstoreRepositoryInterface;
+use Drupal\layout_builder\SectionStorageInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Discards any pending changes to the layout.
+ *
+ * @internal
+ *   Form classes are internal.
+ */
+class DiscardLayoutChangesForm extends ConfirmFormBase implements WorkspaceDynamicSafeFormInterface {
+
+  use WorkspaceSafeFormTrait;
+
+  /**
+   * The layout tempstore repository.
+   *
+   * @var \Drupal\layout_builder\LayoutTempstoreRepositoryInterface
+   */
+  protected $layoutTempstoreRepository;
+
+  /**
+   * The messenger service.
+   *
+   * @var \Drupal\Core\Messenger\MessengerInterface
+   */
+  protected $messenger;
+
+  /**
+   * The section storage.
+   *
+   * @var \Drupal\layout_builder\SectionStorageInterface
+   */
+  protected $sectionStorage;
+
+  /**
+   * Constructs a new DiscardLayoutChangesForm.
+   *
+   * @param \Drupal\layout_builder\LayoutTempstoreRepositoryInterface $layout_tempstore_repository
+   *   The layout tempstore repository.
+   * @param \Drupal\Core\Messenger\MessengerInterface $messenger
+   *   The messenger service.
+   */
+  public function __construct(LayoutTempstoreRepositoryInterface $layout_tempstore_repository, MessengerInterface $messenger) {
+    $this->layoutTempstoreRepository = $layout_tempstore_repository;
+    $this->messenger = $messenger;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('layout_builder.tempstore_repository'),
+      $container->get('messenger')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'layout_builder_discard_changes';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getQuestion() {
+    return $this->t('Are you sure you want to discard your layout changes?');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getCancelUrl() {
+    return $this->sectionStorage->getLayoutBuilderUrl();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state, ?SectionStorageInterface $section_storage = NULL) {
+    $this->sectionStorage = $section_storage;
+    // Mark this as an administrative page for JavaScript ("Back to site" link).
+    $form['#attached']['drupalSettings']['path']['currentPathIsAdmin'] = TRUE;
+    return parent::buildForm($form, $form_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+    $this->layoutTempstoreRepository->delete($this->sectionStorage);
+
+    $this->messenger->addMessage($this->t('The changes to the layout have been discarded.'));
+
+    $form_state->setRedirectUrl($this->sectionStorage->getRedirectUrl());
+  }
+
+}

core/modules/layout_builder/src/Form/LayoutBuilderDisableForm.php

@@ -0,0 +1,114 @@
+<?php
+
+namespace Drupal\layout_builder\Form;
+
+use Drupal\Core\Form\ConfirmFormBase;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Form\WorkspaceDynamicSafeFormInterface;
+use Drupal\Core\Messenger\MessengerInterface;
+use Drupal\layout_builder\DefaultsSectionStorageInterface;
+use Drupal\layout_builder\LayoutTempstoreRepositoryInterface;
+use Drupal\layout_builder\SectionStorageInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Disables Layout Builder for a given default.
+ *
+ * @internal
+ *   Form classes are internal.
+ */
+class LayoutBuilderDisableForm extends ConfirmFormBase implements WorkspaceDynamicSafeFormInterface {
+
+  use WorkspaceSafeFormTrait;
+
+  /**
+   * The layout tempstore repository.
+   *
+   * @var \Drupal\layout_builder\LayoutTempstoreRepositoryInterface
+   */
+  protected $layoutTempstoreRepository;
+
+  /**
+   * The section storage.
+   *
+   * @var \Drupal\layout_builder\DefaultsSectionStorageInterface
+   */
+  protected $sectionStorage;
+
+  /**
+   * Constructs a new RevertOverridesForm.
+   *
+   * @param \Drupal\layout_builder\LayoutTempstoreRepositoryInterface $layout_tempstore_repository
+   *   The layout tempstore repository.
+   * @param \Drupal\Core\Messenger\MessengerInterface $messenger
+   *   The messenger service.
+   */
+  public function __construct(LayoutTempstoreRepositoryInterface $layout_tempstore_repository, MessengerInterface $messenger) {
+    $this->layoutTempstoreRepository = $layout_tempstore_repository;
+    $this->setMessenger($messenger);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('layout_builder.tempstore_repository'),
+      $container->get('messenger')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'layout_builder_disable_form';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getQuestion() {
+    return $this->t('Are you sure you want to disable Layout Builder?');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getDescription() {
+    return $this->t('All customizations will be removed. This action cannot be undone.');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getCancelUrl() {
+    return $this->sectionStorage->getRedirectUrl();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state, ?SectionStorageInterface $section_storage = NULL) {
+    if (!$section_storage instanceof DefaultsSectionStorageInterface) {
+      throw new \InvalidArgumentException(sprintf('The section storage with type "%s" and ID "%s" does not provide defaults', $section_storage->getStorageType(), $section_storage->getStorageId()));
+    }
+
+    $this->sectionStorage = $section_storage;
+    // Mark this as an administrative page for JavaScript ("Back to site" link).
+    $form['#attached']['drupalSettings']['path']['currentPathIsAdmin'] = TRUE;
+    return parent::buildForm($form, $form_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+    $this->sectionStorage->disableLayoutBuilder()->save();
+    $this->layoutTempstoreRepository->delete($this->sectionStorage);
+
+    $this->messenger()->addMessage($this->t('Layout Builder has been disabled.'));
+    $form_state->setRedirectUrl($this->getCancelUrl());
+  }
+
+}

core/modules/layout_builder/src/Form/LayoutBuilderEntityFormTrait.php

@@ -0,0 +1,109 @@
+<?php
+
+namespace Drupal\layout_builder\Form;
+
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\layout_builder\SectionStorageInterface;
+
+/**
+ * Provides a trait for common methods used in Layout Builder entity forms.
+ */
+trait LayoutBuilderEntityFormTrait {
+
+  use PreviewToggleTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getBaseFormId(): string {
+    return $this->getEntity()->getEntityTypeId() . '_layout_builder_form';
+  }
+
+  /**
+   * Build the message container.
+   *
+   * @param \Drupal\Core\StringTranslation\TranslatableMarkup $message
+   *   The message to display.
+   * @param string $type
+   *   The form type this is being attached to.
+   *
+   * @return array
+   *   The render array.
+   */
+  protected function buildMessageContainer(TranslatableMarkup $message, string $type): array {
+    return [
+      '#type' => 'container',
+      '#attributes' => [
+        'class' => [
+          'layout-builder__message',
+          sprintf('layout-builder__message--%s', $type),
+        ],
+      ],
+      'message' => [
+        '#theme' => 'status_messages',
+        '#message_list' => ['status' => [$message]],
+        '#status_headings' => [
+          'status' => $this->t('Status message'),
+        ],
+      ],
+      '#weight' => -900,
+    ];
+  }
+
+  /**
+   * Form submission handler.
+   */
+  public function redirectOnSubmit(array $form, FormStateInterface $form_state) {
+    $form_state->setRedirectUrl($this->sectionStorage->getLayoutBuilderUrl($form_state->getTriggeringElement()['#redirect']));
+  }
+
+  /**
+   * Retrieves the section storage object.
+   *
+   * @return \Drupal\layout_builder\SectionStorageInterface
+   *   The section storage for the current form.
+   */
+  public function getSectionStorage(): SectionStorageInterface {
+    return $this->sectionStorage;
+  }
+
+  /**
+   * Builds the actions for the form.
+   *
+   * @param array $actions
+   *   The actions array to modify.
+   *
+   * @return array
+   *   The modified actions array.
+   */
+  protected function buildActions(array $actions): array {
+    $actions['#attributes']['role'] = 'region';
+    $actions['#attributes']['aria-label'] = $this->t('Layout Builder tools');
+    $actions['submit']['#value'] = $this->t('Save layout');
+    $actions['#weight'] = -1000;
+    $actions['discard_changes'] = [
+      '#type' => 'submit',
+      '#value' => $this->t('Discard changes'),
+      '#submit' => ['::redirectOnSubmit'],
+      '#redirect' => 'discard_changes',
+    ];
+    $actions['preview_toggle'] = $this->buildContentPreviewToggle();
+    return $actions;
+  }
+
+  /**
+   * Performs tasks that are needed during the save process.
+   *
+   * @param \Drupal\Core\Form\FormStateInterface $formState
+   *   The form state.
+   * @param \Drupal\Core\StringTranslation\TranslatableMarkup $message
+   *   The message to display.
+   */
+  protected function saveTasks(FormStateInterface $formState, TranslatableMarkup $message): void {
+    $this->layoutTempstoreRepository->delete($this->getSectionStorage());
+    $this->messenger()->addStatus($message);
+    $formState->setRedirectUrl($this->getSectionStorage()->getRedirectUrl());
+  }
+
+}

core/modules/layout_builder/src/Form/LayoutBuilderEntityViewDisplayForm.php

@@ -0,0 +1,241 @@
+<?php
+
+namespace Drupal\layout_builder\Form;
+
+use Drupal\Core\Entity\EntityInterface;
+use Drupal\Core\Field\FieldDefinitionInterface;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\field_ui\Form\EntityViewDisplayEditForm;
+use Drupal\layout_builder\Entity\LayoutEntityDisplayInterface;
+use Drupal\layout_builder\Plugin\SectionStorage\OverridesSectionStorage;
+use Drupal\layout_builder\SectionStorageInterface;
+
+/**
+ * Edit form for the LayoutBuilderEntityViewDisplay entity type.
+ *
+ * @internal
+ *   Form classes are internal.
+ */
+class LayoutBuilderEntityViewDisplayForm extends EntityViewDisplayEditForm {
+
+  /**
+   * The entity being used by this form.
+   *
+   * @var \Drupal\layout_builder\Entity\LayoutEntityDisplayInterface
+   */
+  protected $entity;
+
+  /**
+   * The storage section.
+   *
+   * @var \Drupal\layout_builder\DefaultsSectionStorageInterface
+   */
+  protected $sectionStorage;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state, ?SectionStorageInterface $section_storage = NULL) {
+    $this->sectionStorage = $section_storage;
+    return parent::buildForm($form, $form_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function form(array $form, FormStateInterface $form_state) {
+    $form = parent::form($form, $form_state);
+
+    // Remove the Layout Builder field from the list.
+    $form['#fields'] = array_diff($form['#fields'], [OverridesSectionStorage::FIELD_NAME]);
+    unset($form['fields'][OverridesSectionStorage::FIELD_NAME]);
+
+    $is_enabled = $this->entity->isLayoutBuilderEnabled();
+    if ($is_enabled) {
+      // Hide the table of fields.
+      $form['fields']['#access'] = FALSE;
+      $form['#fields'] = [];
+      $form['#extra'] = [];
+    }
+
+    $form['manage_layout'] = [
+      '#type' => 'link',
+      '#title' => $this->t('Manage layout'),
+      '#weight' => -10,
+      '#attributes' => ['class' => ['button']],
+      '#url' => $this->sectionStorage->getLayoutBuilderUrl(),
+      '#access' => $is_enabled,
+    ];
+
+    $form['layout'] = [
+      '#type' => 'details',
+      '#open' => TRUE,
+      '#title' => $this->t('Layout options'),
+      '#tree' => TRUE,
+    ];
+
+    $form['layout']['enabled'] = [
+      '#type' => 'checkbox',
+      '#title' => $this->t('Use Layout Builder'),
+      '#default_value' => $is_enabled,
+    ];
+    $form['#entity_builders']['layout_builder'] = '::entityFormEntityBuild';
+
+    // @todo Expand to work for all view modes in
+    //   https://www.drupal.org/node/2907413.
+    if ($this->isCanonicalMode($this->entity->getMode())) {
+      $entity_type = $this->entityTypeManager->getDefinition($this->entity->getTargetEntityTypeId());
+      $form['layout']['allow_custom'] = [
+        '#type' => 'checkbox',
+        '#title' => $this->t('Allow each @entity to have its layout customized.', [
+          '@entity' => $entity_type->getSingularLabel(),
+        ]),
+        '#default_value' => $this->entity->isOverridable(),
+        '#states' => [
+          'disabled' => [
+            ':input[name="layout[enabled]"]' => ['checked' => FALSE],
+          ],
+          'invisible' => [
+            ':input[name="layout[enabled]"]' => ['checked' => FALSE],
+          ],
+        ],
+      ];
+      if (!$is_enabled) {
+        $form['layout']['allow_custom']['#attributes']['disabled'] = 'disabled';
+      }
+      // Prevent turning off overrides while any exist.
+      if ($this->hasOverrides($this->entity)) {
+        $form['layout']['enabled']['#disabled'] = TRUE;
+        $form['layout']['enabled']['#description'] = $this->t('You must revert all customized layouts of this display before you can disable this option.');
+        $form['layout']['allow_custom']['#disabled'] = TRUE;
+        $form['layout']['allow_custom']['#description'] = $this->t('You must revert all customized layouts of this display before you can disable this option.');
+        unset($form['layout']['allow_custom']['#states']);
+        unset($form['#entity_builders']['layout_builder']);
+      }
+    }
+    // For non-canonical modes, the existing value should be preserved.
+    else {
+      $form['layout']['allow_custom'] = [
+        '#type' => 'value',
+        '#value' => $this->entity->isOverridable(),
+      ];
+    }
+    return $form;
+  }
+
+  /**
+   * Determines if the mode is used by the canonical route.
+   *
+   * @param string $mode
+   *   The view mode.
+   *
+   * @return bool
+   *   TRUE if the mode is valid, FALSE otherwise.
+   */
+  protected function isCanonicalMode($mode) {
+    // @todo This is a convention core uses but is not a given, nor is it easily
+    //   introspectable. Address in https://www.drupal.org/node/2907413.
+    $canonical_mode = 'full';
+
+    if ($mode === $canonical_mode) {
+      return TRUE;
+    }
+
+    // The default mode is valid if the canonical mode is not enabled.
+    if ($mode === 'default') {
+      /** @var \Drupal\Core\Config\Entity\ConfigEntityStorageInterface $storage */
+      $storage = $this->entityTypeManager->getStorage($this->entity->getEntityTypeId());
+      $query = $storage->getQuery()
+        ->condition('targetEntityType', $this->entity->getTargetEntityTypeId())
+        ->condition('bundle', $this->entity->getTargetBundle())
+        ->condition('status', TRUE)
+        ->condition('mode', $canonical_mode);
+      return !$query->count()->execute();
+    }
+
+    return FALSE;
+  }
+
+  /**
+   * Determines if the defaults have any overrides.
+   *
+   * @param \Drupal\layout_builder\Entity\LayoutEntityDisplayInterface $display
+   *   The entity display.
+   *
+   * @return bool
+   *   TRUE if there are any overrides of this default, FALSE otherwise.
+   */
+  protected function hasOverrides(LayoutEntityDisplayInterface $display) {
+    if (!$display->isOverridable()) {
+      return FALSE;
+    }
+
+    $entity_type = $this->entityTypeManager->getDefinition($display->getTargetEntityTypeId());
+    $query = $this->entityTypeManager->getStorage($display->getTargetEntityTypeId())->getQuery()
+      ->accessCheck(FALSE)
+      ->exists(OverridesSectionStorage::FIELD_NAME);
+    if ($bundle_key = $entity_type->getKey('bundle')) {
+      $query->condition($bundle_key, $display->getTargetBundle());
+    }
+    return (bool) $query->count()->execute();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function copyFormValuesToEntity(EntityInterface $entity, array $form, FormStateInterface $form_state) {
+    // Do not process field values if Layout Builder is or will be enabled.
+    $set_enabled = (bool) $form_state->getValue(['layout', 'enabled'], FALSE);
+    /** @var \Drupal\layout_builder\Entity\LayoutEntityDisplayInterface $entity */
+    $already_enabled = $entity->isLayoutBuilderEnabled();
+    if ($already_enabled || $set_enabled) {
+      $form['#fields'] = [];
+      $form['#extra'] = [];
+    }
+
+    parent::copyFormValuesToEntity($entity, $form, $form_state);
+  }
+
+  /**
+   * Entity builder for layout options on the entity view display form.
+   */
+  public function entityFormEntityBuild($entity_type_id, LayoutEntityDisplayInterface $display, &$form, FormStateInterface &$form_state) {
+    $set_enabled = (bool) $form_state->getValue(['layout', 'enabled'], FALSE);
+    $already_enabled = $display->isLayoutBuilderEnabled();
+
+    if ($set_enabled) {
+      $overridable = (bool) $form_state->getValue(['layout', 'allow_custom'], FALSE);
+      $display->setOverridable($overridable);
+
+      if (!$already_enabled) {
+        $display->enableLayoutBuilder();
+      }
+    }
+    elseif ($already_enabled) {
+      $form_state->setRedirectUrl($this->sectionStorage->getLayoutBuilderUrl('disable'));
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function buildFieldRow(FieldDefinitionInterface $field_definition, array $form, FormStateInterface $form_state) {
+    if ($this->entity->isLayoutBuilderEnabled()) {
+      return [];
+    }
+
+    return parent::buildFieldRow($field_definition, $form, $form_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function buildExtraFieldRow($field_id, $extra_field) {
+    if ($this->entity->isLayoutBuilderEnabled()) {
+      return [];
+    }
+
+    return parent::buildExtraFieldRow($field_id, $extra_field);
+  }
+
+}

core/modules/layout_builder/src/Form/LayoutRebuildConfirmFormBase.php

@@ -0,0 +1,128 @@
+<?php
+
+namespace Drupal\layout_builder\Form;
+
+use Drupal\Core\Ajax\AjaxFormHelperTrait;
+use Drupal\Core\Form\ConfirmFormBase;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Form\WorkspaceDynamicSafeFormInterface;
+use Drupal\layout_builder\Controller\LayoutRebuildTrait;
+use Drupal\layout_builder\LayoutBuilderHighlightTrait;
+use Drupal\layout_builder\LayoutTempstoreRepositoryInterface;
+use Drupal\layout_builder\SectionStorageInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Provides a base class for confirmation forms that rebuild the Layout Builder.
+ *
+ * @internal
+ *   Form classes are internal.
+ */
+abstract class LayoutRebuildConfirmFormBase extends ConfirmFormBase implements WorkspaceDynamicSafeFormInterface {
+
+  use AjaxFormHelperTrait;
+  use LayoutBuilderHighlightTrait;
+  use LayoutRebuildTrait;
+  use WorkspaceSafeFormTrait;
+
+  /**
+   * The layout tempstore repository.
+   *
+   * @var \Drupal\layout_builder\LayoutTempstoreRepositoryInterface
+   */
+  protected $layoutTempstoreRepository;
+
+  /**
+   * The section storage.
+   *
+   * @var \Drupal\layout_builder\SectionStorageInterface
+   */
+  protected $sectionStorage;
+
+  /**
+   * The field delta.
+   *
+   * @var int
+   */
+  protected $delta;
+
+  /**
+   * Constructs a new RemoveSectionForm.
+   *
+   * @param \Drupal\layout_builder\LayoutTempstoreRepositoryInterface $layout_tempstore_repository
+   *   The layout tempstore repository.
+   */
+  public function __construct(LayoutTempstoreRepositoryInterface $layout_tempstore_repository) {
+    $this->layoutTempstoreRepository = $layout_tempstore_repository;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('layout_builder.tempstore_repository')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getCancelUrl() {
+    return $this->sectionStorage->getLayoutBuilderUrl();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state, ?SectionStorageInterface $section_storage = NULL, $delta = NULL) {
+    $this->sectionStorage = $section_storage;
+    $this->delta = $delta;
+
+    $form = parent::buildForm($form, $form_state);
+
+    if ($this->isAjax()) {
+      $form['actions']['submit']['#ajax']['callback'] = '::ajaxSubmit';
+      $form['actions']['cancel']['#attributes']['class'][] = 'dialog-cancel';
+      $target_highlight_id = !empty($this->uuid) ? $this->blockUpdateHighlightId($this->uuid) : $this->sectionUpdateHighlightId($delta);
+      $form['#attributes']['data-layout-builder-target-highlight-id'] = $target_highlight_id;
+      // The AJAX system automatically moves focus to the first tabbable
+      // element after closing a dialog, sometimes scrolling to a page top.
+      // Disable refocus on the button.
+      $form['actions']['submit']['#ajax']['disable-refocus'] = TRUE;
+    }
+
+    // Mark this as an administrative page for JavaScript ("Back to site" link).
+    $form['#attached']['drupalSettings']['path']['currentPathIsAdmin'] = TRUE;
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+    $this->handleSectionStorage($this->sectionStorage, $form_state);
+
+    $this->layoutTempstoreRepository->set($this->sectionStorage);
+
+    $form_state->setRedirectUrl($this->getCancelUrl());
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function successfulAjaxSubmit(array $form, FormStateInterface $form_state) {
+    return $this->rebuildAndClose($this->sectionStorage);
+  }
+
+  /**
+   * Performs any actions on the section storage before saving.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   */
+  abstract protected function handleSectionStorage(SectionStorageInterface $section_storage, FormStateInterface $form_state);
+
+}

core/modules/layout_builder/src/Form/MoveBlockForm.php

@@ -0,0 +1,345 @@
+<?php
+
+namespace Drupal\layout_builder\Form;
+
+use Drupal\Core\Ajax\AjaxFormHelperTrait;
+use Drupal\Core\Form\FormBase;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Form\WorkspaceDynamicSafeFormInterface;
+use Drupal\layout_builder\Context\LayoutBuilderContextTrait;
+use Drupal\layout_builder\Controller\LayoutRebuildTrait;
+use Drupal\layout_builder\LayoutBuilderHighlightTrait;
+use Drupal\layout_builder\LayoutTempstoreRepositoryInterface;
+use Drupal\layout_builder\SectionStorageInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Provides a form for moving a block.
+ *
+ * @internal
+ *   Form classes are internal.
+ */
+class MoveBlockForm extends FormBase implements WorkspaceDynamicSafeFormInterface {
+
+  use AjaxFormHelperTrait;
+  use LayoutBuilderContextTrait;
+  use LayoutBuilderHighlightTrait;
+  use LayoutRebuildTrait;
+  use WorkspaceSafeFormTrait;
+
+  /**
+   * The section storage.
+   *
+   * @var \Drupal\layout_builder\SectionStorageInterface
+   */
+  protected $sectionStorage;
+
+  /**
+   * The section delta.
+   *
+   * @var int
+   */
+  protected $delta;
+
+  /**
+   * The region name.
+   *
+   * @var string
+   */
+  protected $region;
+
+  /**
+   * The component uuid.
+   *
+   * @var string
+   */
+  protected $uuid;
+
+  /**
+   * The Layout Tempstore.
+   *
+   * @var \Drupal\layout_builder\LayoutTempstoreRepositoryInterface
+   */
+  protected $layoutTempstore;
+
+  /**
+   * Constructs a new MoveBlockForm.
+   *
+   * @param \Drupal\layout_builder\LayoutTempstoreRepositoryInterface $layout_tempstore_repository
+   *   The layout tempstore.
+   */
+  public function __construct(LayoutTempstoreRepositoryInterface $layout_tempstore_repository) {
+    $this->layoutTempstore = $layout_tempstore_repository;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('layout_builder.tempstore_repository')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'layout_builder_block_move';
+  }
+
+  /**
+   * Builds the move block form.
+   *
+   * @param array $form
+   *   An associative array containing the structure of the form.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage being configured.
+   * @param int $delta
+   *   The original delta of the section.
+   * @param string $region
+   *   The original region of the block.
+   * @param string $uuid
+   *   The UUID of the block being updated.
+   *
+   * @return array
+   *   The form array.
+   */
+  public function buildForm(array $form, FormStateInterface $form_state, ?SectionStorageInterface $section_storage = NULL, $delta = NULL, $region = NULL, $uuid = NULL) {
+    $parameters = array_slice(func_get_args(), 2);
+    foreach ($parameters as $parameter) {
+      if (is_null($parameter)) {
+        throw new \InvalidArgumentException('MoveBlockForm requires all parameters.');
+      }
+    }
+
+    $this->sectionStorage = $section_storage;
+    $this->delta = $delta;
+    $this->uuid = $uuid;
+    $this->region = $region;
+
+    $form['#attributes']['data-layout-builder-target-highlight-id'] = $this->blockUpdateHighlightId($uuid);
+
+    $sections = $section_storage->getSections();
+    $contexts = $this->getPopulatedContexts($section_storage);
+    $region_options = [];
+    foreach ($sections as $section_delta => $section) {
+      $layout = $section->getLayout($contexts);
+      $layout_definition = $layout->getPluginDefinition();
+      if (!($section_label = $section->getLayoutSettings()['label'])) {
+        $section_label = $this->t('Section: @delta', ['@delta' => $section_delta + 1])->render();
+      }
+      foreach ($layout_definition->getRegions() as $region_name => $region_info) {
+        // Group regions by section.
+        $region_options[$section_label]["$section_delta:$region_name"] = $this->t(
+          '@section, Region: @region',
+          ['@section' => $section_label, '@region' => $region_info['label']]
+        );
+      }
+    }
+
+    // $this->region and $this->delta are where the block is currently placed.
+    // $selected_region and $selected_delta are the values from this form
+    // specifying where the block should be moved to.
+    $selected_region = $this->getSelectedRegion($form_state);
+    $selected_delta = $this->getSelectedDelta($form_state);
+    $form['region'] = [
+      '#type' => 'select',
+      '#options' => $region_options,
+      '#title' => $this->t('Region'),
+      '#default_value' => "$selected_delta:$selected_region",
+      '#ajax' => [
+        'wrapper' => 'layout-builder-components-table',
+        'callback' => '::getComponentsWrapper',
+      ],
+    ];
+    $current_section = $sections[$selected_delta];
+
+    $aria_label = $this->t('Blocks in Section: @section, Region: @region', ['@section' => $selected_delta + 1, '@region' => $selected_region]);
+
+    $form['components_wrapper']['components'] = [
+      '#type' => 'table',
+      '#header' => [
+        $this->t('Block label'),
+        $this->t('Weight'),
+      ],
+      '#tabledrag' => [
+        [
+          'action' => 'order',
+          'relationship' => 'sibling',
+          'group' => 'table-sort-weight',
+        ],
+      ],
+      // Create a wrapping element so that the Ajax update also replaces the
+      // 'Show block weights' link.
+      '#theme_wrappers' => [
+        'container' => [
+          '#attributes' => [
+            'id' => 'layout-builder-components-table',
+            'class' => ['layout-builder-components-table'],
+            'aria-label' => $aria_label,
+          ],
+        ],
+      ],
+    ];
+
+    /** @var \Drupal\layout_builder\SectionComponent[] $components */
+    $components = $current_section->getComponentsByRegion($selected_region);
+
+    // If the component is not in this region, add it to the listed components.
+    if (!isset($components[$uuid])) {
+      $components[$uuid] = $sections[$delta]->getComponent($uuid);
+    }
+    $state_weight_delta = round(count($components) / 2);
+    foreach ($components as $component_uuid => $component) {
+      /** @var \Drupal\Core\Block\BlockPluginInterface $plugin */
+      $plugin = $component->getPlugin();
+      $is_current_block = $component_uuid === $uuid;
+      $row_classes = [
+        'draggable',
+        'layout-builder-components-table__row',
+      ];
+
+      $label['#wrapper_attributes']['class'] = ['layout-builder-components-table__block-label'];
+
+      if ($is_current_block) {
+        // Highlight the current block.
+        $label['#markup'] = $this->t('@label (current)', ['@label' => $plugin->label()]);
+        $label['#wrapper_attributes']['class'][] = 'layout-builder-components-table__block-label--current';
+        $row_classes[] = 'layout-builder-components-table__row--current';
+      }
+      else {
+        $label['#markup'] = $plugin->label();
+      }
+
+      $form['components_wrapper']['components'][$component_uuid] = [
+        '#attributes' => ['class' => $row_classes],
+        'label' => $label,
+        'weight' => [
+          '#type' => 'weight',
+          '#default_value' => $component->getWeight(),
+          '#title' => $this->t('Weight for @block block', ['@block' => $plugin->label()]),
+          '#title_display' => 'invisible',
+          '#attributes' => [
+            'class' => ['table-sort-weight'],
+          ],
+          '#delta' => $state_weight_delta,
+        ],
+      ];
+    }
+
+    $form['actions']['submit'] = [
+      '#type' => 'submit',
+      '#value' => $this->t('Move'),
+      '#button_type' => 'primary',
+    ];
+
+    $form['#attributes']['data-add-layout-builder-wrapper'] = 'layout-builder--move-blocks-active';
+
+    if ($this->isAjax()) {
+      $form['actions']['submit']['#ajax']['callback'] = '::ajaxSubmit';
+    }
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+    $region = $this->getSelectedRegion($form_state);
+    $delta = $this->getSelectedDelta($form_state);
+    $original_section = $this->sectionStorage->getSection($this->delta);
+    $component = $original_section->getComponent($this->uuid);
+    $section = $this->sectionStorage->getSection($delta);
+    if ($delta !== $this->delta) {
+      // Remove component from old section and add it to the new section.
+      $original_section->removeComponent($this->uuid);
+      $section->insertComponent(0, $component);
+    }
+    $component->setRegion($region);
+    foreach ($form_state->getValue('components') as $uuid => $component_info) {
+      $section->getComponent($uuid)->setWeight($component_info['weight']);
+    }
+    $this->layoutTempstore->set($this->sectionStorage);
+  }
+
+  /**
+   * Ajax callback for the region select element.
+   *
+   * @param array $form
+   *   The form.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The form state.
+   *
+   * @return array
+   *   The components wrapper render array.
+   */
+  public function getComponentsWrapper(array $form, FormStateInterface $form_state) {
+    return $form['components_wrapper'];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function successfulAjaxSubmit(array $form, FormStateInterface $form_state) {
+    return $this->rebuildAndClose($this->sectionStorage);
+  }
+
+  /**
+   * Gets the selected region.
+   *
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The form state.
+   *
+   * @return string
+   *   The current region name.
+   */
+  protected function getSelectedRegion(FormStateInterface $form_state) {
+    if ($form_state->hasValue('region')) {
+      return explode(':', $form_state->getValue('region'), 2)[1];
+    }
+    return $this->region;
+  }
+
+  /**
+   * Gets the selected delta.
+   *
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The form state.
+   *
+   * @return int
+   *   The section delta.
+   */
+  protected function getSelectedDelta(FormStateInterface $form_state) {
+    if ($form_state->hasValue('region')) {
+      return (int) explode(':', $form_state->getValue('region'))[0];
+    }
+    return (int) $this->delta;
+  }
+
+  /**
+   * Provides a title callback.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   * @param int $delta
+   *   The original delta of the section.
+   * @param string $uuid
+   *   The UUID of the block being updated.
+   *
+   * @return string
+   *   The title for the move block form.
+   */
+  public function title(SectionStorageInterface $section_storage, $delta, $uuid) {
+    $block_label = $section_storage
+      ->getSection($delta)
+      ->getComponent($uuid)
+      ->getPlugin()
+      ->label();
+
+    return $this->t('Move the @block_label block', ['@block_label' => $block_label]);
+  }
+
+}

core/modules/layout_builder/src/Form/OverridesEntityForm.php

@@ -0,0 +1,181 @@
+<?php
+
+namespace Drupal\layout_builder\Form;
+
+use Drupal\Component\Datetime\TimeInterface;
+use Drupal\Core\Entity\ContentEntityForm;
+use Drupal\Core\Entity\Entity\EntityFormDisplay;
+use Drupal\Core\Entity\EntityInterface;
+use Drupal\Core\Entity\EntityRepositoryInterface;
+use Drupal\Core\Entity\EntityTypeBundleInfoInterface;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Form\WorkspaceDynamicSafeFormInterface;
+use Drupal\layout_builder\LayoutTempstoreRepositoryInterface;
+use Drupal\layout_builder\OverridesSectionStorageInterface;
+use Drupal\layout_builder\Plugin\SectionStorage\OverridesSectionStorage;
+use Drupal\layout_builder\SectionStorageInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Provides a form containing the Layout Builder UI for overrides.
+ *
+ * @internal
+ *   Form classes are internal.
+ */
+class OverridesEntityForm extends ContentEntityForm implements WorkspaceDynamicSafeFormInterface {
+
+  use PreviewToggleTrait;
+  use LayoutBuilderEntityFormTrait;
+  use WorkspaceSafeFormTrait;
+
+  /**
+   * Layout tempstore repository.
+   *
+   * @var \Drupal\layout_builder\LayoutTempstoreRepositoryInterface
+   */
+  protected $layoutTempstoreRepository;
+
+  /**
+   * The section storage.
+   *
+   * @var \Drupal\layout_builder\SectionStorageInterface
+   */
+  protected $sectionStorage;
+
+  /**
+   * Constructs a new OverridesEntityForm.
+   *
+   * @param \Drupal\Core\Entity\EntityRepositoryInterface $entity_repository
+   *   The entity repository service.
+   * @param \Drupal\Core\Entity\EntityTypeBundleInfoInterface $entity_type_bundle_info
+   *   The entity type bundle service.
+   * @param \Drupal\Component\Datetime\TimeInterface $time
+   *   The time service.
+   * @param \Drupal\layout_builder\LayoutTempstoreRepositoryInterface $layout_tempstore_repository
+   *   The layout tempstore repository.
+   */
+  public function __construct(EntityRepositoryInterface $entity_repository, EntityTypeBundleInfoInterface $entity_type_bundle_info, TimeInterface $time, LayoutTempstoreRepositoryInterface $layout_tempstore_repository) {
+    parent::__construct($entity_repository, $entity_type_bundle_info, $time);
+    $this->layoutTempstoreRepository = $layout_tempstore_repository;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('entity.repository'),
+      $container->get('entity_type.bundle.info'),
+      $container->get('datetime.time'),
+      $container->get('layout_builder.tempstore_repository')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function init(FormStateInterface $form_state) {
+    parent::init($form_state);
+
+    $form_display = EntityFormDisplay::collectRenderDisplay($this->entity, $this->getOperation(), FALSE);
+    $form_display->setComponent(OverridesSectionStorage::FIELD_NAME, [
+      'type' => 'layout_builder_widget',
+      'weight' => -10,
+      'settings' => [],
+    ]);
+
+    $this->setFormDisplay($form_display, $form_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state, ?SectionStorageInterface $section_storage = NULL) {
+    $this->sectionStorage = $section_storage;
+    $form = parent::buildForm($form, $form_state);
+    $form['#attributes']['class'][] = 'layout-builder-form';
+
+    // @todo \Drupal\layout_builder\Field\LayoutSectionItemList::defaultAccess()
+    //   restricts all access to the field, explicitly allow access here until
+    //   https://www.drupal.org/node/2942975 is resolved.
+    $form[OverridesSectionStorage::FIELD_NAME]['#access'] = TRUE;
+
+    $form['layout_builder_message'] = $this->buildMessage($section_storage->getContextValue('entity'), $section_storage);
+    return $form;
+  }
+
+  /**
+   * Renders a message to display at the top of the layout builder.
+   *
+   * @param \Drupal\Core\Entity\EntityInterface $entity
+   *   The entity whose layout is being edited.
+   * @param \Drupal\layout_builder\OverridesSectionStorageInterface $section_storage
+   *   The current section storage.
+   *
+   * @return array
+   *   A renderable array containing the message.
+   */
+  protected function buildMessage(EntityInterface $entity, OverridesSectionStorageInterface $section_storage) {
+    $entity_type = $entity->getEntityType();
+    $bundle_info = $this->entityTypeBundleInfo->getBundleInfo($entity->getEntityTypeId());
+
+    $variables = [
+      '@bundle' => $bundle_info[$entity->bundle()]['label'],
+      '@singular_label' => $entity_type->getSingularLabel(),
+      '@plural_label' => $entity_type->getPluralLabel(),
+    ];
+
+    $defaults_link = $section_storage
+      ->getDefaultSectionStorage()
+      ->getLayoutBuilderUrl();
+
+    if ($defaults_link->access($this->currentUser())) {
+      $variables[':link'] = $defaults_link->toString();
+      if ($entity_type->hasKey('bundle')) {
+        $message = $this->t('You are editing the layout for this @bundle @singular_label. <a href=":link">Edit the template for all @bundle @plural_label instead.</a>', $variables);
+      }
+      else {
+        $message = $this->t('You are editing the layout for this @singular_label. <a href=":link">Edit the template for all @plural_label instead.</a>', $variables);
+      }
+    }
+    else {
+      if ($entity_type->hasKey('bundle')) {
+        $message = $this->t('You are editing the layout for this @bundle @singular_label.', $variables);
+      }
+      else {
+        $message = $this->t('You are editing the layout for this @singular_label.', $variables);
+      }
+    }
+    return $this->buildMessageContainer($message, 'overrides');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function save(array $form, FormStateInterface $form_state) {
+    $return = parent::save($form, $form_state);
+    $this->saveTasks($form_state, $this->t('The layout override has been saved.'));
+    return $return;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function actions(array $form, FormStateInterface $form_state) {
+    $actions = parent::actions($form, $form_state);
+    $actions = $this->buildActions($actions);
+    $actions['delete']['#access'] = FALSE;
+
+    $actions['discard_changes']['#limit_validation_errors'] = [];
+    // @todo This button should be conditionally displayed, see
+    //   https://www.drupal.org/node/2917777.
+    $actions['revert'] = [
+      '#type' => 'submit',
+      '#value' => $this->t('Revert to defaults'),
+      '#submit' => ['::redirectOnSubmit'],
+      '#redirect' => 'revert',
+    ];
+    return $actions;
+  }
+
+}

core/modules/layout_builder/src/Form/PreviewToggleTrait.php

@@ -0,0 +1,43 @@
+<?php
+
+namespace Drupal\layout_builder\Form;
+
+/**
+ * Provides a trait that provides a toggle for the content preview.
+ */
+trait PreviewToggleTrait {
+
+  /**
+   * Builds the content preview toggle input.
+   *
+   * @return array
+   *   The render array for the content preview toggle.
+   */
+  protected function buildContentPreviewToggle() {
+    return [
+      '#type' => 'container',
+      '#attributes' => [
+        'class' => ['js-show'],
+      ],
+      'toggle_content_preview' => [
+        '#title' => $this->t('Show content preview'),
+        '#type' => 'checkbox',
+        '#value' => TRUE,
+        '#attributes' => [
+          // Set attribute used by local storage to get content preview status.
+          'data-content-preview-id' => "Drupal.layout_builder.content_preview.{$this->currentUser()->id()}",
+        ],
+        '#id' => 'layout-builder-content-preview',
+      ],
+    ];
+  }
+
+  /**
+   * Gets the current user.
+   *
+   * @return \Drupal\Core\Session\AccountInterface
+   *   The current user.
+   */
+  abstract protected function currentUser();
+
+}

core/modules/layout_builder/src/Form/RemoveBlockForm.php

@@ -0,0 +1,73 @@
+<?php
+
+namespace Drupal\layout_builder\Form;
+
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\layout_builder\SectionStorageInterface;
+
+/**
+ * Provides a form to confirm the removal of a block.
+ *
+ * @internal
+ *   Form classes are internal.
+ */
+class RemoveBlockForm extends LayoutRebuildConfirmFormBase {
+
+  /**
+   * The current region.
+   *
+   * @var string
+   */
+  protected $region;
+
+  /**
+   * The UUID of the block being removed.
+   *
+   * @var string
+   */
+  protected $uuid;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getQuestion() {
+    $label = $this->sectionStorage
+      ->getSection($this->delta)
+      ->getComponent($this->uuid)
+      ->getPlugin()
+      ->label();
+
+    return $this->t('Are you sure you want to remove the %label block?', ['%label' => $label]);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getConfirmText() {
+    return $this->t('Remove');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'layout_builder_remove_block';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state, ?SectionStorageInterface $section_storage = NULL, $delta = NULL, $region = NULL, $uuid = NULL) {
+    $this->region = $region;
+    $this->uuid = $uuid;
+    return parent::buildForm($form, $form_state, $section_storage, $delta);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function handleSectionStorage(SectionStorageInterface $section_storage, FormStateInterface $form_state) {
+    $section_storage->getSection($this->delta)->removeComponent($this->uuid);
+  }
+
+}

core/modules/layout_builder/src/Form/RemoveSectionForm.php

@@ -0,0 +1,50 @@
+<?php
+
+namespace Drupal\layout_builder\Form;
+
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\layout_builder\SectionStorageInterface;
+
+/**
+ * Provides a form to confirm the removal of a section.
+ *
+ * @internal
+ *   Form classes are internal.
+ */
+class RemoveSectionForm extends LayoutRebuildConfirmFormBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'layout_builder_remove_section';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getQuestion() {
+    $configuration = $this->sectionStorage->getSection($this->delta)->getLayoutSettings();
+    // Layouts may choose to use a class that might not have a label
+    // configuration.
+    if (!empty($configuration['label'])) {
+      return $this->t('Are you sure you want to remove @section?', ['@section' => $configuration['label']]);
+    }
+    return $this->t('Are you sure you want to remove section @section?', ['@section' => $this->delta + 1]);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getConfirmText() {
+    return $this->t('Remove');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function handleSectionStorage(SectionStorageInterface $section_storage, FormStateInterface $form_state) {
+    $section_storage->removeSection($this->delta);
+  }
+
+}

core/modules/layout_builder/src/Form/RevertOverridesForm.php

@@ -0,0 +1,124 @@
+<?php
+
+namespace Drupal\layout_builder\Form;
+
+use Drupal\Core\Form\ConfirmFormBase;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Form\WorkspaceDynamicSafeFormInterface;
+use Drupal\Core\Messenger\MessengerInterface;
+use Drupal\layout_builder\LayoutTempstoreRepositoryInterface;
+use Drupal\layout_builder\OverridesSectionStorageInterface;
+use Drupal\layout_builder\SectionStorageInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Reverts the overridden layout to the defaults.
+ *
+ * @internal
+ *   Form classes are internal.
+ */
+class RevertOverridesForm extends ConfirmFormBase implements WorkspaceDynamicSafeFormInterface {
+
+  use WorkspaceSafeFormTrait;
+
+  /**
+   * The layout tempstore repository.
+   *
+   * @var \Drupal\layout_builder\LayoutTempstoreRepositoryInterface
+   */
+  protected $layoutTempstoreRepository;
+
+  /**
+   * The messenger service.
+   *
+   * @var \Drupal\Core\Messenger\MessengerInterface
+   */
+  protected $messenger;
+
+  /**
+   * The section storage.
+   *
+   * @var \Drupal\layout_builder\SectionStorageInterface
+   */
+  protected $sectionStorage;
+
+  /**
+   * Constructs a new RevertOverridesForm.
+   *
+   * @param \Drupal\layout_builder\LayoutTempstoreRepositoryInterface $layout_tempstore_repository
+   *   The layout tempstore repository.
+   * @param \Drupal\Core\Messenger\MessengerInterface $messenger
+   *   The messenger service.
+   */
+  public function __construct(LayoutTempstoreRepositoryInterface $layout_tempstore_repository, MessengerInterface $messenger) {
+    $this->layoutTempstoreRepository = $layout_tempstore_repository;
+    $this->messenger = $messenger;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('layout_builder.tempstore_repository'),
+      $container->get('messenger')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'layout_builder_revert_overrides';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getQuestion() {
+    return $this->t('Are you sure you want to revert this to defaults?');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getConfirmText() {
+    return $this->t('Revert');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getCancelUrl() {
+    return $this->sectionStorage->getLayoutBuilderUrl();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state, ?SectionStorageInterface $section_storage = NULL) {
+    if (!$section_storage instanceof OverridesSectionStorageInterface) {
+      throw new \InvalidArgumentException(sprintf('The section storage with type "%s" and ID "%s" does not provide overrides', $section_storage->getStorageType(), $section_storage->getStorageId()));
+    }
+
+    $this->sectionStorage = $section_storage;
+    // Mark this as an administrative page for JavaScript ("Back to site" link).
+    $form['#attached']['drupalSettings']['path']['currentPathIsAdmin'] = TRUE;
+    return parent::buildForm($form, $form_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+    // Remove all sections.
+    $this->sectionStorage
+      ->removeAllSections()
+      ->save();
+    $this->layoutTempstoreRepository->delete($this->sectionStorage);
+
+    $this->messenger->addMessage($this->t('The layout has been reverted back to defaults.'));
+    $form_state->setRedirectUrl($this->sectionStorage->getRedirectUrl());
+  }
+
+}

core/modules/layout_builder/src/Form/UpdateBlockForm.php

@@ -0,0 +1,58 @@
+<?php
+
+namespace Drupal\layout_builder\Form;
+
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\layout_builder\LayoutBuilderHighlightTrait;
+use Drupal\layout_builder\SectionStorageInterface;
+
+/**
+ * Provides a form to update a block.
+ *
+ * @internal
+ *   Form classes are internal.
+ */
+class UpdateBlockForm extends ConfigureBlockFormBase {
+
+  use LayoutBuilderHighlightTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'layout_builder_update_block';
+  }
+
+  /**
+   * Builds the block form.
+   *
+   * @param array $form
+   *   An associative array containing the structure of the form.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage being configured.
+   * @param int $delta
+   *   The delta of the section.
+   * @param string $region
+   *   The region of the block.
+   * @param string $uuid
+   *   The UUID of the block being updated.
+   *
+   * @return array
+   *   The form array.
+   */
+  public function buildForm(array $form, FormStateInterface $form_state, ?SectionStorageInterface $section_storage = NULL, $delta = NULL, $region = NULL, $uuid = NULL) {
+    $component = $section_storage->getSection($delta)->getComponent($uuid);
+    $form['#attributes']['data-layout-builder-target-highlight-id'] = $this->blockUpdateHighlightId($uuid);
+    return $this->doBuildForm($form, $form_state, $section_storage, $delta, $component);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function submitLabel() {
+    return $this->t('Update');
+  }
+
+}

core/modules/layout_builder/src/Form/WorkspaceSafeFormTrait.php

@@ -0,0 +1,63 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\layout_builder\Form;
+
+use Drupal\Core\Entity\Form\WorkspaceSafeFormTrait as EntityWorkspaceSafeFormTrait;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\layout_builder\SectionStorageInterface;
+
+/**
+ * Provides a trait that marks Layout Builder forms as workspace-safe.
+ */
+trait WorkspaceSafeFormTrait {
+
+  use EntityWorkspaceSafeFormTrait;
+
+  /**
+   * Determines whether the current form is safe to be submitted in a workspace.
+   *
+   * @param array $form
+   *   An associative array containing the structure of the form.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   *
+   * @return bool
+   *   TRUE if the form is workspace-safe, FALSE otherwise.
+   */
+  public function isWorkspaceSafeForm(array $form, FormStateInterface $form_state): bool {
+    $section_storage = $this->sectionStorage ?: $this->getSectionStorageFromFormState($form_state);
+    if ($section_storage) {
+      $context_definitions = $section_storage->getContextDefinitions();
+      if (!empty($context_definitions['entity'])) {
+        /** @var \Drupal\Core\Entity\EntityInterface $entity */
+        $entity = $section_storage->getContextValue('entity');
+
+        return $this->isWorkspaceSafeEntity($entity);
+      }
+    }
+
+    return FALSE;
+  }
+
+  /**
+   * Retrieves the section storage from a form state object, if it exists.
+   *
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The form state object.
+   *
+   * @return \Drupal\layout_builder\SectionStorageInterface|null
+   *   The section storage or NULL if it doesn't exist.
+   */
+  protected function getSectionStorageFromFormState(FormStateInterface $form_state): ?SectionStorageInterface {
+    foreach ($form_state->getBuildInfo()['args'] as $argument) {
+      if ($argument instanceof SectionStorageInterface) {
+        return $argument;
+      }
+    }
+
+    return NULL;
+  }
+
+}

core/modules/layout_builder/src/InlineBlockEntityOperations.php

@@ -0,0 +1,241 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
+use Drupal\Core\Entity\EntityInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Entity\RevisionableInterface;
+use Drupal\Core\Entity\SynchronizableInterface;
+use Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Defines a class for reacting to entity events related to Inline Blocks.
+ *
+ * @internal
+ *   This is an internal utility class wrapping hook implementations.
+ */
+class InlineBlockEntityOperations implements ContainerInjectionInterface {
+
+  use LayoutEntityHelperTrait;
+
+  /**
+   * Inline block usage tracking service.
+   *
+   * @var \Drupal\layout_builder\InlineBlockUsageInterface
+   */
+  protected $usage;
+
+  /**
+   * The block content storage.
+   *
+   * @var \Drupal\Core\Entity\EntityStorageInterface
+   */
+  protected $blockContentStorage;
+
+  /**
+   * The entity type manager.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
+   */
+  protected $entityTypeManager;
+
+  /**
+   * Constructs a new EntityOperations object.
+   *
+   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entityTypeManager
+   *   The entity type manager service.
+   * @param \Drupal\layout_builder\InlineBlockUsageInterface $usage
+   *   Inline block usage tracking service.
+   * @param \Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface $section_storage_manager
+   *   The section storage manager.
+   */
+  public function __construct(EntityTypeManagerInterface $entityTypeManager, InlineBlockUsageInterface $usage, SectionStorageManagerInterface $section_storage_manager) {
+    $this->entityTypeManager = $entityTypeManager;
+    $this->blockContentStorage = $entityTypeManager->getStorage('block_content');
+    $this->usage = $usage;
+    $this->sectionStorageManager = $section_storage_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('entity_type.manager'),
+      $container->get('inline_block.usage'),
+      $container->get('plugin.manager.layout_builder.section_storage')
+    );
+  }
+
+  /**
+   * Remove all unused inline blocks on save.
+   *
+   * Entities that were used in prevision revisions will be removed if not
+   * saving a new revision.
+   *
+   * @param \Drupal\Core\Entity\EntityInterface $entity
+   *   The parent entity.
+   */
+  protected function removeUnusedForEntityOnSave(EntityInterface $entity) {
+    // If the entity is new or '$entity->original' is not set then there will
+    // not be any unused inline blocks to remove.
+    // If this is a revisionable entity then do not remove inline blocks. They
+    // could be referenced in previous revisions even if this is not a new
+    // revision.
+    if ($entity->isNew() || !isset($entity->original) || $entity instanceof RevisionableInterface) {
+      return;
+    }
+    // If the original entity used the default storage then we cannot remove
+    // unused inline blocks because they will still be referenced in the
+    // defaults.
+    if ($this->originalEntityUsesDefaultStorage($entity)) {
+      return;
+    }
+
+    // Delete and remove the usage for inline blocks that were removed.
+    if ($removed_block_ids = $this->getRemovedBlockIds($entity)) {
+      $this->deleteBlocksAndUsage($removed_block_ids);
+    }
+  }
+
+  /**
+   * Gets the IDs of the inline blocks that were removed.
+   *
+   * @param \Drupal\Core\Entity\EntityInterface $entity
+   *   The layout entity.
+   *
+   * @return int[]
+   *   The block content IDs that were removed.
+   */
+  protected function getRemovedBlockIds(EntityInterface $entity) {
+    $original_sections = $this->getEntitySections($entity->original);
+    $current_sections = $this->getEntitySections($entity);
+    // Avoid un-needed conversion from revision IDs to block content IDs by
+    // first determining if there are any revisions in the original that are not
+    // also in the current sections.
+    $current_block_content_revision_ids = $this->getInlineBlockRevisionIdsInSections($current_sections);
+    $original_block_content_revision_ids = $this->getInlineBlockRevisionIdsInSections($original_sections);
+    if ($unused_original_revision_ids = array_diff($original_block_content_revision_ids, $current_block_content_revision_ids)) {
+      // If there are any revisions in the original that aren't in the current
+      // there may some blocks that need to be removed.
+      $current_block_content_ids = $this->getBlockIdsForRevisionIds($current_block_content_revision_ids);
+      $unused_original_block_content_ids = $this->getBlockIdsForRevisionIds($unused_original_revision_ids);
+      return array_diff($unused_original_block_content_ids, $current_block_content_ids);
+    }
+    return [];
+  }
+
+  /**
+   * Handles entity tracking on deleting a parent entity.
+   *
+   * @param \Drupal\Core\Entity\EntityInterface $entity
+   *   The parent entity.
+   */
+  public function handleEntityDelete(EntityInterface $entity) {
+    // @todo In https://www.drupal.org/node/3008943 call
+    //   \Drupal\layout_builder\LayoutEntityHelperTrait::isLayoutCompatibleEntity().
+    $this->usage->removeByLayoutEntity($entity);
+  }
+
+  /**
+   * Handles saving a parent entity.
+   *
+   * @param \Drupal\Core\Entity\EntityInterface $entity
+   *   The parent entity.
+   */
+  public function handlePreSave(EntityInterface $entity) {
+    if (($entity instanceof SynchronizableInterface && $entity->isSyncing())
+      || !$this->isLayoutCompatibleEntity($entity)
+    ) {
+      return;
+    }
+
+    $duplicate_blocks = FALSE;
+    if ($sections = $this->getEntitySections($entity)) {
+      if ($this->originalEntityUsesDefaultStorage($entity)) {
+        // This is a new override from a default and the blocks need to be
+        // duplicated.
+        $duplicate_blocks = TRUE;
+      }
+      // Since multiple parent entity revisions may reference common block
+      // revisions, when a block is modified, it must always result in the
+      // creation of a new block revision.
+      $new_revision = $entity instanceof RevisionableInterface;
+      foreach ($this->getInlineBlockComponents($sections) as $component) {
+        $this->saveInlineBlockComponent($entity, $component, $new_revision, $duplicate_blocks);
+      }
+    }
+    $this->removeUnusedForEntityOnSave($entity);
+  }
+
+  /**
+   * Delete the inline blocks and the usage records.
+   *
+   * @param int[] $block_content_ids
+   *   The block content entity IDs.
+   */
+  protected function deleteBlocksAndUsage(array $block_content_ids) {
+    foreach ($block_content_ids as $block_content_id) {
+      if ($block = $this->blockContentStorage->load($block_content_id)) {
+        $block->delete();
+      }
+    }
+    $this->usage->deleteUsage($block_content_ids);
+  }
+
+  /**
+   * Removes unused inline blocks.
+   *
+   * @param int $limit
+   *   The maximum number of inline blocks to remove.
+   */
+  public function removeUnused($limit = 100) {
+    $this->deleteBlocksAndUsage($this->usage->getUnused($limit));
+  }
+
+  /**
+   * Gets blocks IDs for an array of revision IDs.
+   *
+   * @param int[] $revision_ids
+   *   The revision IDs.
+   *
+   * @return int[]
+   *   The block IDs.
+   */
+  protected function getBlockIdsForRevisionIds(array $revision_ids) {
+    if ($revision_ids) {
+      $query = $this->blockContentStorage->getQuery()->accessCheck(FALSE);
+      $query->condition('revision_id', $revision_ids, 'IN');
+      $block_ids = $query->execute();
+      return $block_ids;
+    }
+    return [];
+  }
+
+  /**
+   * Saves an inline block component.
+   *
+   * @param \Drupal\Core\Entity\EntityInterface $entity
+   *   The entity with the layout.
+   * @param \Drupal\layout_builder\SectionComponent $component
+   *   The section component with an inline block.
+   * @param bool $new_revision
+   *   Whether a new revision of the block should be created when modified.
+   * @param bool $duplicate_blocks
+   *   Whether the blocks should be duplicated.
+   */
+  protected function saveInlineBlockComponent(EntityInterface $entity, SectionComponent $component, $new_revision, $duplicate_blocks) {
+    /** @var \Drupal\layout_builder\Plugin\Block\InlineBlock $plugin */
+    $plugin = $component->getPlugin();
+    $pre_save_configuration = $plugin->getConfiguration();
+    $plugin->saveBlockContent($new_revision, $duplicate_blocks);
+    $post_save_configuration = $plugin->getConfiguration();
+    if ($duplicate_blocks || (empty($pre_save_configuration['block_revision_id']) && !empty($post_save_configuration['block_revision_id']))) {
+      $this->usage->addUsage($post_save_configuration['block_id'], $entity);
+    }
+    $component->setConfiguration($post_save_configuration);
+  }
+
+}

core/modules/layout_builder/src/InlineBlockUsage.php

@@ -0,0 +1,88 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+use Drupal\Core\Database\Connection;
+use Drupal\Core\Entity\EntityInterface;
+
+/**
+ * Service class to track inline block usage.
+ */
+class InlineBlockUsage implements InlineBlockUsageInterface {
+
+  /**
+   * The database connection.
+   *
+   * @var \Drupal\Core\Database\Connection
+   */
+  protected $database;
+
+  /**
+   * Creates an InlineBlockUsage object.
+   *
+   * @param \Drupal\Core\Database\Connection $database
+   *   The database connection.
+   */
+  public function __construct(Connection $database) {
+    $this->database = $database;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function addUsage($block_content_id, EntityInterface $entity) {
+    $this->database->merge('inline_block_usage')
+      ->keys([
+        'block_content_id' => $block_content_id,
+        'layout_entity_id' => $entity->id(),
+        'layout_entity_type' => $entity->getEntityTypeId(),
+      ])->execute();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getUnused($limit = 100) {
+    $query = $this->database->select('inline_block_usage', 't');
+    $query->fields('t', ['block_content_id']);
+    $query->isNull('layout_entity_id');
+    $query->isNull('layout_entity_type');
+    return $query->range(0, $limit)->execute()->fetchCol();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function removeByLayoutEntity(EntityInterface $entity) {
+    $query = $this->database->update('inline_block_usage')
+      ->fields([
+        'layout_entity_type' => NULL,
+        'layout_entity_id' => NULL,
+      ]);
+    $query->condition('layout_entity_type', $entity->getEntityTypeId());
+    $query->condition('layout_entity_id', $entity->id());
+    $query->execute();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function deleteUsage(array $block_content_ids) {
+    if (!empty($block_content_ids)) {
+      $query = $this->database->delete('inline_block_usage')->condition('block_content_id', $block_content_ids, 'IN');
+      $query->execute();
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getUsage($block_content_id) {
+    $query = $this->database->select('inline_block_usage');
+    $query->condition('block_content_id', $block_content_id);
+    $query->fields('inline_block_usage', ['layout_entity_id', 'layout_entity_type']);
+    $query->range(0, 1);
+    return $query->execute()->fetchObject();
+  }
+
+}

core/modules/layout_builder/src/InlineBlockUsageInterface.php

@@ -0,0 +1,61 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+use Drupal\Core\Entity\EntityInterface;
+
+/**
+ * Defines an interface for tracking inline block usage.
+ */
+interface InlineBlockUsageInterface {
+
+  /**
+   * Adds a usage record.
+   *
+   * @param int $block_content_id
+   *   The block content ID.
+   * @param \Drupal\Core\Entity\EntityInterface $entity
+   *   The layout entity.
+   */
+  public function addUsage($block_content_id, EntityInterface $entity);
+
+  /**
+   * Gets unused inline block IDs.
+   *
+   * @param int $limit
+   *   The maximum number of block content entity IDs to return.
+   *
+   * @return int[]
+   *   The entity IDs.
+   */
+  public function getUnused($limit = 100);
+
+  /**
+   * Remove usage record by layout entity.
+   *
+   * @param \Drupal\Core\Entity\EntityInterface $entity
+   *   The layout entity.
+   */
+  public function removeByLayoutEntity(EntityInterface $entity);
+
+  /**
+   * Delete the inline blocks' the usage records.
+   *
+   * @param int[] $block_content_ids
+   *   The block content entity IDs.
+   */
+  public function deleteUsage(array $block_content_ids);
+
+  /**
+   * Gets usage record for inline block by ID.
+   *
+   * @param int $block_content_id
+   *   The block content entity ID.
+   *
+   * @return object|false
+   *   The usage record with properties layout_entity_id and layout_entity_type
+   *   or FALSE if there is no usage.
+   */
+  public function getUsage($block_content_id);
+
+}

core/modules/layout_builder/src/LayoutBuilderEnabledInterface.php

@@ -0,0 +1,32 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+/**
+ * Provides methods for enabling and disabling Layout Builder.
+ */
+interface LayoutBuilderEnabledInterface {
+
+  /**
+   * Determines if Layout Builder is enabled.
+   *
+   * @return bool
+   *   TRUE if Layout Builder is enabled, FALSE otherwise.
+   */
+  public function isLayoutBuilderEnabled();
+
+  /**
+   * Enables the Layout Builder.
+   *
+   * @return $this
+   */
+  public function enableLayoutBuilder();
+
+  /**
+   * Disables the Layout Builder.
+   *
+   * @return $this
+   */
+  public function disableLayoutBuilder();
+
+}

core/modules/layout_builder/src/LayoutBuilderEvents.php

@@ -0,0 +1,42 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+/**
+ * Defines events for the layout_builder module.
+ *
+ * @see \Drupal\layout_builder\Event\SectionComponentBuildRenderArrayEvent
+ */
+final class LayoutBuilderEvents {
+
+  /**
+   * Name of the event fired when a component's render array is built.
+   *
+   * This event allows modules to collaborate on creating the render array of
+   * the SectionComponent object. The event listener method receives a
+   * \Drupal\layout_builder\Event\SectionComponentBuildRenderArrayEvent
+   * instance.
+   *
+   * @Event
+   *
+   * @see \Drupal\layout_builder\Event\SectionComponentBuildRenderArrayEvent
+   * @see \Drupal\layout_builder\SectionComponent::toRenderArray()
+   *
+   * @var string
+   */
+  const SECTION_COMPONENT_BUILD_RENDER_ARRAY = 'section_component.build.render_array';
+
+  /**
+   * Name of the event fired in when preparing a layout builder element.
+   *
+   * This event allows modules to collaborate on creating the sections used in
+   * \Drupal\layout_builder\Element\LayoutBuilder during #pre_render.
+   *
+   * @see \Drupal\layout_builder\Event\PrepareLayoutEvent
+   * @see \Drupal\layout_builder\Element\LayoutBuilder
+   *
+   * @var string
+   */
+  const PREPARE_LAYOUT = 'prepare_layout';
+
+}

core/modules/layout_builder/src/LayoutBuilderHighlightTrait.php

@@ -0,0 +1,64 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+/**
+ * A trait for generating IDs used to highlight active UI elements.
+ */
+trait LayoutBuilderHighlightTrait {
+
+  /**
+   * Provides the ID used to highlight the active Layout Builder UI element.
+   *
+   * @param string $delta
+   *   The section the block is in.
+   * @param string $region
+   *   The section region in which the block is placed.
+   *
+   * @return string
+   *   The highlight ID of the block.
+   */
+  protected function blockAddHighlightId($delta, $region) {
+    return "block-$delta-$region";
+  }
+
+  /**
+   * Provides the ID used to highlight the active Layout Builder UI element.
+   *
+   * @param string $uuid
+   *   The uuid of the block.
+   *
+   * @return string
+   *   The highlight ID of the block.
+   */
+  protected function blockUpdateHighlightId($uuid) {
+    return $uuid;
+  }
+
+  /**
+   * Provides the ID used to highlight the active Layout Builder UI element.
+   *
+   * @param string $delta
+   *   The location of the section.
+   *
+   * @return string
+   *   The highlight ID of the section.
+   */
+  protected function sectionAddHighlightId($delta) {
+    return "section-$delta";
+  }
+
+  /**
+   * Provides the ID used to highlight the active Layout Builder UI element.
+   *
+   * @param string $delta
+   *   The location of the section.
+   *
+   * @return string
+   *   The highlight ID of the section.
+   */
+  protected function sectionUpdateHighlightId($delta) {
+    return "section-update-$delta";
+  }
+
+}

core/modules/layout_builder/src/LayoutBuilderOverridableInterface.php

@@ -0,0 +1,28 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+/**
+ * Provides an interface for displays that could be overridable.
+ */
+interface LayoutBuilderOverridableInterface {
+
+  /**
+   * Determines if the display allows custom overrides.
+   *
+   * @return bool
+   *   TRUE if custom overrides are allowed, FALSE otherwise.
+   */
+  public function isOverridable();
+
+  /**
+   * Sets the display to allow or disallow overrides.
+   *
+   * @param bool $overridable
+   *   TRUE if the display should allow overrides, FALSE otherwise.
+   *
+   * @return $this
+   */
+  public function setOverridable($overridable = TRUE);
+
+}

core/modules/layout_builder/src/LayoutBuilderOverridesPermissions.php

@@ -0,0 +1,115 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
+use Drupal\Core\Entity\EntityTypeBundleInfoInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Provides dynamic permissions for Layout Builder overrides.
+ *
+ * @see \Drupal\layout_builder\Plugin\SectionStorage\OverridesSectionStorage::access()
+ *
+ * @internal
+ *   Dynamic permission callbacks are internal.
+ */
+class LayoutBuilderOverridesPermissions implements ContainerInjectionInterface {
+
+  use StringTranslationTrait;
+
+  /**
+   * The entity type manager.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
+   */
+  protected $entityTypeManager;
+
+  /**
+   * The entity type bundle info service.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeBundleInfoInterface
+   */
+  protected $bundleInfo;
+
+  /**
+   * LayoutBuilderOverridesPermissions constructor.
+   *
+   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
+   *   The entity type manager.
+   * @param \Drupal\Core\Entity\EntityTypeBundleInfoInterface $bundle_info
+   *   The bundle info service.
+   */
+  public function __construct(EntityTypeManagerInterface $entity_type_manager, EntityTypeBundleInfoInterface $bundle_info) {
+    $this->entityTypeManager = $entity_type_manager;
+    $this->bundleInfo = $bundle_info;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('entity_type.manager'),
+      $container->get('entity_type.bundle.info')
+    );
+  }
+
+  /**
+   * Returns an array of permissions.
+   *
+   * @return string[][]
+   *   An array whose keys are permission names and whose corresponding values
+   *   are defined in \Drupal\user\PermissionHandlerInterface::getPermissions().
+   */
+  public function permissions() {
+    $permissions = [];
+
+    /** @var \Drupal\layout_builder\Entity\LayoutEntityDisplayInterface[] $entity_displays */
+    $entity_displays = $this->entityTypeManager->getStorage('entity_view_display')->loadByProperties(['third_party_settings.layout_builder.allow_custom' => TRUE]);
+    foreach ($entity_displays as $entity_display) {
+      $entity_type_id = $entity_display->getTargetEntityTypeId();
+      $entity_type = $this->entityTypeManager->getDefinition($entity_type_id);
+      $bundle = $entity_display->getTargetBundle();
+      $args = [
+        '%entity_type' => $entity_type->getCollectionLabel(),
+        '@entity_type_singular' => $entity_type->getSingularLabel(),
+        '@entity_type_plural' => $entity_type->getPluralLabel(),
+        '%bundle' => $this->bundleInfo->getBundleInfo($entity_type_id)[$bundle]['label'],
+      ];
+      // These permissions are generated on behalf of $entity_display entity
+      // display, therefore add this entity display as a config dependency.
+      $dependencies = [
+        $entity_display->getConfigDependencyKey() => [
+          $entity_display->getConfigDependencyName(),
+        ],
+      ];
+      if ($entity_type->hasKey('bundle')) {
+        $permissions["configure all $bundle $entity_type_id layout overrides"] = [
+          'title' => $this->t('%entity_type - %bundle: Configure all layout overrides', $args),
+          'warning' => $this->t('Warning: Allows configuring the layout even if the user cannot edit the @entity_type_singular itself.', $args),
+          'dependencies' => $dependencies,
+        ];
+        $permissions["configure editable $bundle $entity_type_id layout overrides"] = [
+          'title' => $this->t('%entity_type - %bundle: Configure layout overrides for @entity_type_plural that the user can edit', $args),
+          'dependencies' => $dependencies,
+        ];
+      }
+      else {
+        $permissions["configure all $bundle $entity_type_id layout overrides"] = [
+          'title' => $this->t('%entity_type: Configure all layout overrides', $args),
+          'warning' => $this->t('Warning: Allows configuring the layout even if the user cannot edit the @entity_type_singular itself.', $args),
+          'dependencies' => $dependencies,
+        ];
+        $permissions["configure editable $bundle $entity_type_id layout overrides"] = [
+          'title' => $this->t('%entity_type: Configure layout overrides for @entity_type_plural that the user can edit', $args),
+          'dependencies' => $dependencies,
+        ];
+      }
+    }
+    return $permissions;
+  }
+
+}

core/modules/layout_builder/src/LayoutBuilderServiceProvider.php

@@ -0,0 +1,53 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+use Drupal\Core\DependencyInjection\ContainerBuilder;
+use Drupal\Core\DependencyInjection\ServiceProviderInterface;
+use Drupal\layout_builder\EventSubscriber\SetInlineBlockDependency;
+use Drupal\layout_builder\Normalizer\LayoutEntityDisplayNormalizer;
+use Symfony\Component\DependencyInjection\ChildDefinition;
+use Symfony\Component\DependencyInjection\Definition;
+use Symfony\Component\DependencyInjection\Reference;
+
+/**
+ * Sets the layout_builder.get_block_dependency_subscriber service definition.
+ *
+ * This service is dependent on the block_content module so it must be provided
+ * dynamically.
+ *
+ * @internal
+ *   Service providers are internal.
+ *
+ * @see \Drupal\layout_builder\EventSubscriber\SetInlineBlockDependency
+ */
+class LayoutBuilderServiceProvider implements ServiceProviderInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function register(ContainerBuilder $container) {
+    $modules = $container->getParameter('container.modules');
+    if (isset($modules['block_content'])) {
+      $definition = new Definition(SetInlineBlockDependency::class);
+      $definition->setArguments([
+        new Reference('entity_type.manager'),
+        new Reference('database'),
+        new Reference('inline_block.usage'),
+        new Reference('plugin.manager.layout_builder.section_storage'),
+      ]);
+      $definition->addTag('event_subscriber');
+      $definition->setPublic(TRUE);
+      $container->setDefinition('layout_builder.get_block_dependency_subscriber', $definition);
+    }
+    if (isset($modules['serialization'])) {
+      $definition = (new ChildDefinition('serializer.normalizer.config_entity'))
+        ->setClass(LayoutEntityDisplayNormalizer::class)
+        // Ensure that this normalizer takes precedence for Layout Builder data
+        // over the generic serializer.normalizer.config_entity.
+        ->addTag('normalizer', ['priority' => 5]);
+      $container->setDefinition('layout_builder.normalizer.layout_entity_display', $definition);
+    }
+  }
+
+}

core/modules/layout_builder/src/LayoutEntityHelperTrait.php

@@ -0,0 +1,158 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+use Drupal\Component\Plugin\DerivativeInspectionInterface;
+use Drupal\Core\Cache\CacheableMetadata;
+use Drupal\Core\Entity\Entity\EntityViewDisplay;
+use Drupal\Core\Entity\EntityInterface;
+use Drupal\Core\Entity\FieldableEntityInterface;
+use Drupal\Core\Plugin\Context\Context;
+use Drupal\Core\Plugin\Context\ContextDefinition;
+use Drupal\Core\Plugin\Context\EntityContext;
+use Drupal\layout_builder\Entity\LayoutEntityDisplayInterface;
+
+/**
+ * Methods to help with entities using the layout builder.
+ */
+trait LayoutEntityHelperTrait {
+
+  /**
+   * The section storage manager.
+   *
+   * @var \Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface
+   */
+  protected $sectionStorageManager;
+
+  /**
+   * Determines if an entity can have a layout.
+   *
+   * @param \Drupal\Core\Entity\EntityInterface $entity
+   *   The entity to check.
+   *
+   * @return bool
+   *   TRUE if the entity can have a layout otherwise FALSE.
+   */
+  protected function isLayoutCompatibleEntity(EntityInterface $entity) {
+    return $this->getSectionStorageForEntity($entity) !== NULL;
+  }
+
+  /**
+   * Gets revision IDs for layout sections.
+   *
+   * @param \Drupal\layout_builder\Section[] $sections
+   *   The layout sections.
+   *
+   * @return int[]
+   *   The revision IDs.
+   */
+  protected function getInlineBlockRevisionIdsInSections(array $sections) {
+    $revision_ids = [];
+    foreach ($this->getInlineBlockComponents($sections) as $component) {
+      $configuration = $component->getPlugin()->getConfiguration();
+      if (!empty($configuration['block_revision_id'])) {
+        $revision_ids[] = $configuration['block_revision_id'];
+      }
+    }
+    return $revision_ids;
+  }
+
+  /**
+   * Gets the sections for an entity if any.
+   *
+   * @param \Drupal\Core\Entity\EntityInterface $entity
+   *   The entity.
+   *
+   * @return \Drupal\layout_builder\Section[]
+   *   The entity layout sections if available.
+   */
+  protected function getEntitySections(EntityInterface $entity) {
+    $section_storage = $this->getSectionStorageForEntity($entity);
+    return $section_storage ? $section_storage->getSections() : [];
+  }
+
+  /**
+   * Gets components that have Inline Block plugins.
+   *
+   * @param \Drupal\layout_builder\Section[] $sections
+   *   The layout sections.
+   *
+   * @return \Drupal\layout_builder\SectionComponent[]
+   *   The components that contain Inline Block plugins.
+   */
+  protected function getInlineBlockComponents(array $sections) {
+    $inline_block_components = [];
+    foreach ($sections as $section) {
+      foreach ($section->getComponents() as $component) {
+        $plugin = $component->getPlugin();
+        if ($plugin instanceof DerivativeInspectionInterface && $plugin->getBaseId() === 'inline_block') {
+          $inline_block_components[] = $component;
+        }
+      }
+    }
+    return $inline_block_components;
+  }
+
+  /**
+   * Gets the section storage for an entity.
+   *
+   * @param \Drupal\Core\Entity\EntityInterface $entity
+   *   The entity.
+   *
+   * @return \Drupal\layout_builder\SectionStorageInterface|null
+   *   The section storage if found otherwise NULL.
+   */
+  protected function getSectionStorageForEntity(EntityInterface $entity) {
+    // @todo Take into account other view modes in
+    //   https://www.drupal.org/node/3008924.
+    $view_mode = 'full';
+    if ($entity instanceof LayoutEntityDisplayInterface) {
+      $contexts['display'] = EntityContext::fromEntity($entity);
+      $contexts['view_mode'] = new Context(new ContextDefinition('string'), $entity->getMode());
+    }
+    else {
+      $contexts['entity'] = EntityContext::fromEntity($entity);
+      if ($entity instanceof FieldableEntityInterface) {
+        $display = EntityViewDisplay::collectRenderDisplay($entity, $view_mode);
+        if ($display instanceof LayoutEntityDisplayInterface) {
+          $contexts['display'] = EntityContext::fromEntity($display);
+        }
+        $contexts['view_mode'] = new Context(new ContextDefinition('string'), $view_mode);
+      }
+    }
+    return $this->sectionStorageManager()->findByContext($contexts, new CacheableMetadata());
+  }
+
+  /**
+   * Determines if the original entity used the default section storage.
+   *
+   * This method can be used during the entity save process to determine whether
+   * $entity->original is set and used the default section storage plugin as
+   * determined by ::getSectionStorageForEntity().
+   *
+   * @param \Drupal\Core\Entity\EntityInterface $entity
+   *   The entity.
+   *
+   * @return bool
+   *   TRUE if the original entity used the default storage.
+   */
+  protected function originalEntityUsesDefaultStorage(EntityInterface $entity) {
+    $section_storage = $this->getSectionStorageForEntity($entity);
+    if ($section_storage instanceof OverridesSectionStorageInterface && !$entity->isNew() && isset($entity->original)) {
+      $original_section_storage = $this->getSectionStorageForEntity($entity->original);
+      return $original_section_storage instanceof DefaultsSectionStorageInterface;
+    }
+    return FALSE;
+  }
+
+  /**
+   * Gets the section storage manager.
+   *
+   * @return \Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface
+   *   The section storage manager.
+   */
+  private function sectionStorageManager() {
+    return $this->sectionStorageManager ?: \Drupal::service('plugin.manager.layout_builder.section_storage');
+  }
+
+}

core/modules/layout_builder/src/LayoutTempstoreRepository.php

@@ -0,0 +1,130 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+use Drupal\Core\TempStore\SharedTempStoreFactory;
+
+/**
+ * Provides a mechanism for loading layouts from tempstore.
+ */
+class LayoutTempstoreRepository implements LayoutTempstoreRepositoryInterface {
+
+  /**
+   * The shared tempstore factory.
+   *
+   * @var \Drupal\Core\TempStore\SharedTempStoreFactory
+   */
+  protected $tempStoreFactory;
+
+  /**
+   * The static cache of loaded values.
+   *
+   * @var \Drupal\layout_builder\SectionStorageInterface[]
+   */
+  protected array $cache = [];
+
+  /**
+   * LayoutTempstoreRepository constructor.
+   *
+   * @param \Drupal\Core\TempStore\SharedTempStoreFactory $temp_store_factory
+   *   The shared tempstore factory.
+   */
+  public function __construct(SharedTempStoreFactory $temp_store_factory) {
+    $this->tempStoreFactory = $temp_store_factory;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function get(SectionStorageInterface $section_storage) {
+    $key = $this->getKey($section_storage);
+
+    // Check if the storage is present in the static cache.
+    if (isset($this->cache[$key])) {
+      return $this->cache[$key];
+    }
+
+    $tempstore = $this->getTempstore($section_storage)->get($key);
+    if (!empty($tempstore['section_storage'])) {
+      $storage_type = $section_storage->getStorageType();
+      $section_storage = $tempstore['section_storage'];
+
+      if (!($section_storage instanceof SectionStorageInterface)) {
+        throw new \UnexpectedValueException(sprintf('The entry with storage type "%s" and ID "%s" is invalid', $storage_type, $key));
+      }
+
+      // Set the storage in the static cache.
+      $this->cache[$key] = $section_storage;
+    }
+    return $section_storage;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function has(SectionStorageInterface $section_storage) {
+    $key = $this->getKey($section_storage);
+
+    // Check if the storage is present in the static cache.
+    if (isset($this->cache[$key])) {
+      return TRUE;
+    }
+
+    $tempstore = $this->getTempstore($section_storage)->get($key);
+    return !empty($tempstore['section_storage']);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function set(SectionStorageInterface $section_storage) {
+    $key = $this->getKey($section_storage);
+    $this->getTempstore($section_storage)->set($key, ['section_storage' => $section_storage]);
+    // Update the storage in the static cache.
+    $this->cache[$key] = $section_storage;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function delete(SectionStorageInterface $section_storage) {
+    $key = $this->getKey($section_storage);
+    $this->getTempstore($section_storage)->delete($key);
+    // Remove the storage from the static cache.
+    unset($this->cache[$key]);
+  }
+
+  /**
+   * Gets the shared tempstore.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   *
+   * @return \Drupal\Core\TempStore\SharedTempStore
+   *   The tempstore.
+   */
+  protected function getTempstore(SectionStorageInterface $section_storage) {
+    $collection = 'layout_builder.section_storage.' . $section_storage->getStorageType();
+    return $this->tempStoreFactory->get($collection);
+  }
+
+  /**
+   * Gets the string to use as the tempstore key.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   *
+   * @return string
+   *   A unique string representing the section storage. This should include as
+   *   much identifying information as possible about this particular storage,
+   *   including information like the current language.
+   */
+  protected function getKey(SectionStorageInterface $section_storage) {
+    if ($section_storage instanceof TempStoreIdentifierInterface) {
+      return $section_storage->getTempstoreKey();
+    }
+
+    return $section_storage->getStorageId();
+  }
+
+}

core/modules/layout_builder/src/LayoutTempstoreRepositoryInterface.php

@@ -0,0 +1,52 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+/**
+ * Provides an interface for loading layouts from tempstore.
+ */
+interface LayoutTempstoreRepositoryInterface {
+
+  /**
+   * Gets the tempstore version of a section storage, if it exists.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage to check for in tempstore.
+   *
+   * @return \Drupal\layout_builder\SectionStorageInterface
+   *   Either the version of this section storage from tempstore, or the passed
+   *   section storage if none exists.
+   *
+   * @throw \UnexpectedValueException
+   *   Thrown if a value exists, but is not a section storage.
+   */
+  public function get(SectionStorageInterface $section_storage);
+
+  /**
+   * Stores this section storage in tempstore.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage to set in tempstore.
+   */
+  public function set(SectionStorageInterface $section_storage);
+
+  /**
+   * Checks for the existence of a tempstore version of a section storage.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage to check for in tempstore.
+   *
+   * @return bool
+   *   TRUE if there is a tempstore version of this section storage.
+   */
+  public function has(SectionStorageInterface $section_storage);
+
+  /**
+   * Removes the tempstore version of a section storage.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage to remove from tempstore.
+   */
+  public function delete(SectionStorageInterface $section_storage);
+
+}

core/modules/layout_builder/src/Normalizer/LayoutEntityDisplayNormalizer.php

@@ -0,0 +1,37 @@
+<?php
+
+namespace Drupal\layout_builder\Normalizer;
+
+use Drupal\layout_builder\Entity\LayoutEntityDisplayInterface;
+use Drupal\serialization\Normalizer\ConfigEntityNormalizer;
+
+/**
+ * Normalizes/denormalizes LayoutEntityDisplay objects into an array structure.
+ *
+ * @internal
+ *   Tagged services are internal.
+ */
+class LayoutEntityDisplayNormalizer extends ConfigEntityNormalizer {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static function getDataWithoutInternals(array $data) {
+    $data = parent::getDataWithoutInternals($data);
+    // Do not expose the actual layout sections in normalization.
+    // @todo Determine what to expose here in
+    //   https://www.drupal.org/node/2942975.
+    unset($data['third_party_settings']['layout_builder']['sections']);
+    return $data;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getSupportedTypes(?string $format): array {
+    return [
+      LayoutEntityDisplayInterface::class => TRUE,
+    ];
+  }
+
+}

core/modules/layout_builder/src/OverridesSectionStorageInterface.php

@@ -0,0 +1,29 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+/**
+ * Defines an interface for an object that stores layout sections for overrides.
+ */
+interface OverridesSectionStorageInterface extends SectionStorageInterface {
+
+  /**
+   * Returns the corresponding defaults section storage for this override.
+   *
+   * @return \Drupal\layout_builder\DefaultsSectionStorageInterface
+   *   The defaults section storage.
+   *
+   * @todo Determine if this method needs a parameter in
+   *   https://www.drupal.org/project/drupal/issues/2907413.
+   */
+  public function getDefaultSectionStorage();
+
+  /**
+   * Indicates if overrides are in use.
+   *
+   * @return bool
+   *   TRUE if this overrides section storage is in use, otherwise FALSE.
+   */
+  public function isOverridden();
+
+}

core/modules/layout_builder/src/Plugin/Block/ExtraFieldBlock.php

@@ -0,0 +1,188 @@
+<?php
+
+namespace Drupal\layout_builder\Plugin\Block;
+
+use Drupal\Core\Block\Attribute\Block;
+use Drupal\Core\Block\BlockBase;
+use Drupal\Core\Cache\CacheableMetadata;
+use Drupal\Core\Entity\EntityFieldManagerInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
+use Drupal\Core\Plugin\ContextAwarePluginInterface;
+use Drupal\Core\Render\Element;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\layout_builder\Plugin\Derivative\ExtraFieldBlockDeriver;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Provides a block that renders an extra field from an entity.
+ *
+ * This block handles fields that are provided by implementations of
+ * hook_entity_extra_field_info().
+ *
+ * @see \Drupal\layout_builder\Plugin\Block\FieldBlock
+ *   This block plugin handles all other field entities not provided by
+ *   hook_entity_extra_field_info().
+ *
+ * @internal
+ *   Plugin classes are internal.
+ */
+#[Block(
+  id: "extra_field_block",
+  deriver: ExtraFieldBlockDeriver::class
+)]
+class ExtraFieldBlock extends BlockBase implements ContextAwarePluginInterface, ContainerFactoryPluginInterface {
+
+  /**
+   * The entity field manager.
+   *
+   * @var \Drupal\Core\Entity\EntityFieldManagerInterface
+   */
+  protected $entityFieldManager;
+
+  /**
+   * The field name.
+   *
+   * @var string
+   */
+  protected $fieldName;
+
+  /**
+   * The entity type manager.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
+   */
+  protected $entityTypeManager;
+
+  /**
+   * Constructs a new ExtraFieldBlock.
+   *
+   * @param array $configuration
+   *   A configuration array containing information about the plugin instance.
+   * @param string $plugin_id
+   *   The plugin ID for the plugin instance.
+   * @param mixed $plugin_definition
+   *   The plugin implementation definition.
+   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
+   *   The entity type manager.
+   * @param \Drupal\Core\Entity\EntityFieldManagerInterface $entity_field_manager
+   *   The entity field manager.
+   */
+  public function __construct(array $configuration, $plugin_id, $plugin_definition, EntityTypeManagerInterface $entity_type_manager, EntityFieldManagerInterface $entity_field_manager) {
+    $this->entityTypeManager = $entity_type_manager;
+    $this->entityFieldManager = $entity_field_manager;
+    // Get field name from the plugin ID.
+    [, , , $field_name] = explode(static::DERIVATIVE_SEPARATOR, $plugin_id, 4);
+    assert(!empty($field_name));
+    $this->fieldName = $field_name;
+
+    parent::__construct($configuration, $plugin_id, $plugin_definition);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function defaultConfiguration() {
+    return [
+      'label_display' => FALSE,
+      'formatter' => [
+        'settings' => [],
+        'third_party_settings' => [],
+      ],
+    ];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
+    return new static(
+      $configuration,
+      $plugin_id,
+      $plugin_definition,
+      $container->get('entity_type.manager'),
+      $container->get('entity_field.manager')
+    );
+  }
+
+  /**
+   * Gets the entity that has the field.
+   *
+   * @return \Drupal\Core\Entity\FieldableEntityInterface
+   *   The entity.
+   */
+  protected function getEntity() {
+    return $this->getContextValue('entity');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function build() {
+    $entity = $this->getEntity();
+    // Add a placeholder to replace after the entity view is built.
+    // @see layout_builder_entity_view_alter().
+    $extra_fields = $this->entityFieldManager->getExtraFields($entity->getEntityTypeId(), $entity->bundle());
+    if (!isset($extra_fields['display'][$this->fieldName])) {
+      $build = [];
+    }
+    else {
+      $build = [
+        '#extra_field_placeholder_field_name' => $this->fieldName,
+        // Always provide a placeholder. The Layout Builder will NOT invoke
+        // hook_entity_view_alter() so extra fields will not be added to the
+        // render array. If the hook is invoked the placeholder will be
+        // replaced.
+        // @see ::replaceFieldPlaceholder()
+        '#markup' => $this->t('Placeholder for the @preview_fallback', ['@preview_fallback' => $this->getPreviewFallbackString()]),
+      ];
+    }
+    CacheableMetadata::createFromObject($this)->applyTo($build);
+    return $build;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getPreviewFallbackString() {
+    $entity = $this->getEntity();
+    $extra_fields = $this->entityFieldManager->getExtraFields($entity->getEntityTypeId(), $entity->bundle());
+    return new TranslatableMarkup('"@field" field', ['@field' => $extra_fields['display'][$this->fieldName]['label']]);
+  }
+
+  /**
+   * Replaces all placeholders for a given field.
+   *
+   * @param array $build
+   *   The built render array for the elements.
+   * @param array $built_field
+   *   The render array to replace the placeholder.
+   * @param string $field_name
+   *   The field name.
+   *
+   * @see ::build()
+   */
+  public static function replaceFieldPlaceholder(array &$build, array $built_field, $field_name) {
+    foreach (Element::children($build) as $child) {
+      if (isset($build[$child]['#extra_field_placeholder_field_name']) && $build[$child]['#extra_field_placeholder_field_name'] === $field_name) {
+        $placeholder_cache = CacheableMetadata::createFromRenderArray($build[$child]);
+        $built_cache = CacheableMetadata::createFromRenderArray($built_field);
+        $merged_cache = $placeholder_cache->merge($built_cache);
+        $build[$child] = $built_field;
+        $merged_cache->applyTo($build);
+      }
+      else {
+        static::replaceFieldPlaceholder($build[$child], $built_field, $field_name);
+      }
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function blockAccess(AccountInterface $account) {
+    return $this->getEntity()->access('view', $account, TRUE);
+  }
+
+}

core/modules/layout_builder/src/Plugin/Block/FieldBlock.php

@@ -0,0 +1,441 @@
+<?php
+
+namespace Drupal\layout_builder\Plugin\Block;
+
+use Drupal\Component\Plugin\Factory\DefaultFactory;
+use Drupal\Component\Utility\NestedArray;
+use Drupal\Core\Access\AccessResult;
+use Drupal\Core\Block\Attribute\Block;
+use Drupal\Core\Block\BlockBase;
+use Drupal\Core\Cache\CacheableMetadata;
+use Drupal\Core\Entity\EntityDisplayBase;
+use Drupal\Core\Entity\EntityFieldManagerInterface;
+use Drupal\Core\Entity\FieldableEntityInterface;
+use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Core\Field\FieldDefinitionInterface;
+use Drupal\Core\Field\FormatterInterface;
+use Drupal\Core\Field\FormatterPluginManager;
+use Drupal\Core\Form\EnforcedResponseException;
+use Drupal\Core\Form\FormHelper;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
+use Drupal\Core\Plugin\ContextAwarePluginInterface;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\field\FieldConfigInterface;
+use Drupal\layout_builder\Plugin\Derivative\FieldBlockDeriver;
+use Psr\Log\LoggerInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+use Drupal\field\FieldLabelOptionsTrait;
+
+/**
+ * Provides a block that renders a field from an entity.
+ *
+ * @internal
+ *   Plugin classes are internal.
+ */
+#[Block(
+  id: "field_block",
+  deriver: FieldBlockDeriver::class
+)]
+class FieldBlock extends BlockBase implements ContextAwarePluginInterface, ContainerFactoryPluginInterface {
+
+  use FieldLabelOptionsTrait;
+  /**
+   * The entity field manager.
+   *
+   * @var \Drupal\Core\Entity\EntityFieldManagerInterface
+   */
+  protected $entityFieldManager;
+
+  /**
+   * The formatter manager.
+   *
+   * @var \Drupal\Core\Field\FormatterPluginManager
+   */
+  protected $formatterManager;
+
+  /**
+   * The entity type ID.
+   *
+   * @var string
+   */
+  protected $entityTypeId;
+
+  /**
+   * The bundle ID.
+   *
+   * @var string
+   */
+  protected $bundle;
+
+  /**
+   * The field name.
+   *
+   * @var string
+   */
+  protected $fieldName;
+
+  /**
+   * The field definition.
+   *
+   * @var \Drupal\Core\Field\FieldDefinitionInterface
+   */
+  protected $fieldDefinition;
+
+  /**
+   * The module handler.
+   *
+   * @var \Drupal\Core\Extension\ModuleHandlerInterface
+   */
+  protected $moduleHandler;
+
+  /**
+   * The logger.
+   *
+   * @var \Psr\Log\LoggerInterface
+   */
+  protected $logger;
+
+  /**
+   * Constructs a new FieldBlock.
+   *
+   * @param array $configuration
+   *   A configuration array containing information about the plugin instance.
+   * @param string $plugin_id
+   *   The plugin ID for the plugin instance.
+   * @param mixed $plugin_definition
+   *   The plugin implementation definition.
+   * @param \Drupal\Core\Entity\EntityFieldManagerInterface $entity_field_manager
+   *   The entity field manager.
+   * @param \Drupal\Core\Field\FormatterPluginManager $formatter_manager
+   *   The formatter manager.
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
+   *   The module handler.
+   * @param \Psr\Log\LoggerInterface $logger
+   *   The logger.
+   */
+  public function __construct(array $configuration, $plugin_id, $plugin_definition, EntityFieldManagerInterface $entity_field_manager, FormatterPluginManager $formatter_manager, ModuleHandlerInterface $module_handler, LoggerInterface $logger) {
+    $this->entityFieldManager = $entity_field_manager;
+    $this->formatterManager = $formatter_manager;
+    $this->moduleHandler = $module_handler;
+    $this->logger = $logger;
+
+    // Get the entity type and field name from the plugin ID.
+    [, $entity_type_id, $bundle, $field_name] = explode(static::DERIVATIVE_SEPARATOR, $plugin_id, 4);
+    $this->entityTypeId = $entity_type_id;
+    $this->bundle = $bundle;
+    $this->fieldName = $field_name;
+
+    parent::__construct($configuration, $plugin_id, $plugin_definition);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
+    return new static(
+      $configuration,
+      $plugin_id,
+      $plugin_definition,
+      $container->get('entity_field.manager'),
+      $container->get('plugin.manager.field.formatter'),
+      $container->get('module_handler'),
+      $container->get('logger.channel.layout_builder')
+    );
+  }
+
+  /**
+   * Gets the entity that has the field.
+   *
+   * @return \Drupal\Core\Entity\FieldableEntityInterface
+   *   The entity.
+   */
+  protected function getEntity() {
+    return $this->getContextValue('entity');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function build() {
+    $display_settings = $this->getConfiguration()['formatter'];
+    $display_settings['third_party_settings']['layout_builder']['view_mode'] = $this->getContextValue('view_mode');
+    $entity = $this->getEntity();
+    try {
+      $build = [];
+      $view = $entity->get($this->fieldName)->view($display_settings);
+      if ($view) {
+        $build = [$view];
+      }
+    }
+    // @todo Remove in https://www.drupal.org/project/drupal/issues/2367555.
+    catch (EnforcedResponseException $e) {
+      throw $e;
+    }
+    catch (\Exception $e) {
+      $build = [];
+      $this->logger->warning('The field "%field" failed to render with the error of "%error".', ['%field' => $this->fieldName, '%error' => $e->getMessage()]);
+    }
+    CacheableMetadata::createFromRenderArray($build)->addCacheableDependency($this)->applyTo($build);
+    return $build;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getPreviewFallbackString() {
+    return new TranslatableMarkup('"@field" field', ['@field' => $this->getFieldDefinition()->getLabel()]);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function blockAccess(AccountInterface $account) {
+    $entity = $this->getEntity();
+
+    // First consult the entity.
+    $access = $entity->access('view', $account, TRUE);
+    if (!$access->isAllowed()) {
+      return $access;
+    }
+
+    // Check that the entity in question has this field.
+    if (!$entity instanceof FieldableEntityInterface || !$entity->hasField($this->fieldName)) {
+      return $access->andIf(AccessResult::forbidden());
+    }
+
+    // Check field access.
+    $field = $entity->get($this->fieldName);
+    $access = $access->andIf($field->access('view', $account, TRUE));
+    if (!$access->isAllowed()) {
+      return $access;
+    }
+
+    // Check to see if the field has any values or a default value.
+    if ($field->isEmpty() && !$this->entityFieldHasDefaultValue()) {
+      return $access->andIf(AccessResult::forbidden());
+    }
+    return $access;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function defaultConfiguration() {
+    return [
+      'label_display' => FALSE,
+      'formatter' => [
+        'label' => 'above',
+        'type' => $this->pluginDefinition['default_formatter'],
+        'settings' => [],
+        'third_party_settings' => [],
+      ],
+    ];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function blockForm($form, FormStateInterface $form_state) {
+    $config = $this->getConfiguration();
+
+    $form['formatter'] = [
+      '#tree' => TRUE,
+      '#process' => [
+        [$this, 'formatterSettingsProcessCallback'],
+      ],
+    ];
+    $form['formatter']['label'] = [
+      '#type' => 'select',
+      '#title' => $this->t('Label'),
+      '#options' => $this->getFieldLabelOptions(),
+      '#default_value' => $config['formatter']['label'],
+    ];
+
+    $form['formatter']['type'] = [
+      '#type' => 'select',
+      '#title' => $this->t('Formatter'),
+      '#options' => $this->getApplicablePluginOptions($this->getFieldDefinition()),
+      '#required' => TRUE,
+      '#default_value' => $config['formatter']['type'],
+      '#ajax' => [
+        'callback' => [static::class, 'formatterSettingsAjaxCallback'],
+        'wrapper' => 'formatter-settings-wrapper',
+      ],
+    ];
+
+    // Add the formatter settings to the form via AJAX.
+    $form['formatter']['settings_wrapper'] = [
+      '#prefix' => '<div id="formatter-settings-wrapper">',
+      '#suffix' => '</div>',
+    ];
+
+    return $form;
+  }
+
+  /**
+   * Render API callback: builds the formatter settings elements.
+   */
+  public function formatterSettingsProcessCallback(array &$element, FormStateInterface $form_state, array &$complete_form) {
+    if ($formatter = $this->getFormatter($element['#parents'], $form_state)) {
+      $element['settings_wrapper']['settings'] = $formatter->settingsForm($complete_form, $form_state);
+      $element['settings_wrapper']['settings']['#parents'] = array_merge($element['#parents'], ['settings']);
+      $element['settings_wrapper']['third_party_settings'] = $this->thirdPartySettingsForm($formatter, $this->getFieldDefinition(), $complete_form, $form_state);
+      $element['settings_wrapper']['third_party_settings']['#parents'] = array_merge($element['#parents'], ['third_party_settings']);
+      FormHelper::rewriteStatesSelector($element['settings_wrapper'], "fields[$this->fieldName][settings_edit_form]", 'settings[formatter]');
+
+      // Store the array parents for our element so that we can retrieve the
+      // formatter settings in our AJAX callback.
+      $form_state->set('field_block_array_parents', $element['#array_parents']);
+    }
+    return $element;
+  }
+
+  /**
+   * Adds the formatter third party settings forms.
+   *
+   * @param \Drupal\Core\Field\FormatterInterface $plugin
+   *   The formatter.
+   * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
+   *   The field definition.
+   * @param array $form
+   *   The (entire) configuration form array.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The form state.
+   *
+   * @return array
+   *   The formatter third party settings form.
+   */
+  protected function thirdPartySettingsForm(FormatterInterface $plugin, FieldDefinitionInterface $field_definition, array $form, FormStateInterface $form_state) {
+    $settings_form = [];
+    // Invoke hook_field_formatter_third_party_settings_form(), keying resulting
+    // subforms by module name.
+    $this->moduleHandler->invokeAllWith(
+      'field_formatter_third_party_settings_form',
+      function (callable $hook, string $module) use (&$settings_form, $plugin, $field_definition, $form, $form_state) {
+        $settings_form[$module] = $hook(
+          $plugin,
+          $field_definition,
+          EntityDisplayBase::CUSTOM_MODE,
+          $form,
+          $form_state,
+        );
+      }
+    );
+    return $settings_form;
+  }
+
+  /**
+   * Render API callback: gets the layout settings elements.
+   */
+  public static function formatterSettingsAjaxCallback(array $form, FormStateInterface $form_state) {
+    $formatter_array_parents = $form_state->get('field_block_array_parents');
+    return NestedArray::getValue($form, array_merge($formatter_array_parents, ['settings_wrapper']));
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function blockSubmit($form, FormStateInterface $form_state) {
+    $this->configuration['formatter'] = $form_state->getValue('formatter');
+  }
+
+  /**
+   * Gets the field definition.
+   *
+   * @return \Drupal\Core\Field\FieldDefinitionInterface
+   *   The field definition.
+   */
+  protected function getFieldDefinition() {
+    if (empty($this->fieldDefinition)) {
+      $field_definitions = $this->entityFieldManager->getFieldDefinitions($this->entityTypeId, $this->bundle);
+      $this->fieldDefinition = $field_definitions[$this->fieldName];
+    }
+    return $this->fieldDefinition;
+  }
+
+  /**
+   * Returns an array of applicable formatter options for a field.
+   *
+   * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
+   *   The field definition.
+   *
+   * @return array
+   *   An array of applicable formatter options.
+   *
+   * @see \Drupal\field_ui\Form\EntityDisplayFormBase::getApplicablePluginOptions()
+   */
+  protected function getApplicablePluginOptions(FieldDefinitionInterface $field_definition) {
+    $options = $this->formatterManager->getOptions($field_definition->getType());
+    $applicable_options = [];
+    foreach ($options as $option => $label) {
+      $plugin_class = DefaultFactory::getPluginClass($option, $this->formatterManager->getDefinition($option));
+      if ($plugin_class::isApplicable($field_definition)) {
+        $applicable_options[$option] = $label;
+      }
+    }
+    return $applicable_options;
+  }
+
+  /**
+   * Gets the formatter object.
+   *
+   * @param array $parents
+   *   The #parents of the element representing the formatter.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   *
+   * @return \Drupal\Core\Field\FormatterInterface
+   *   The formatter object.
+   */
+  protected function getFormatter(array $parents, FormStateInterface $form_state) {
+    // Use the processed values, if available.
+    $configuration = NestedArray::getValue($form_state->getValues(), $parents);
+    if (!$configuration) {
+      // Next check the raw user input.
+      $configuration = NestedArray::getValue($form_state->getUserInput(), $parents);
+      if (!$configuration) {
+        // If no user input exists, use the default values.
+        $configuration = $this->getConfiguration()['formatter'];
+      }
+    }
+
+    return $this->formatterManager->getInstance([
+      'configuration' => $configuration,
+      'field_definition' => $this->getFieldDefinition(),
+      'view_mode' => EntityDisplayBase::CUSTOM_MODE,
+      'prepare' => TRUE,
+    ]);
+  }
+
+  /**
+   * Checks whether there is a default value set on the field.
+   *
+   * @return bool
+   *   TRUE if default value set, FALSE otherwise.
+   */
+  protected function entityFieldHasDefaultValue(): bool {
+    $entity = $this->getEntity();
+    $field = $entity->get($this->fieldName);
+    $definition = $field->getFieldDefinition();
+    if ($definition->getDefaultValue($entity)) {
+      return TRUE;
+    }
+
+    // @todo Remove special handling of image fields after
+    // https://www.drupal.org/project/drupal/issues/3005528.
+    if ($definition->getType() !== 'image') {
+      return FALSE;
+    }
+
+    $default_image = $definition->getSetting('default_image');
+    // If we are dealing with a configurable field, look in both instance-level
+    // and field-level settings.
+    if (empty($default_image['uuid']) && ($definition instanceof FieldConfigInterface)) {
+      $default_image = $definition->getFieldStorageDefinition()->getSetting('default_image');
+    }
+    return !empty($default_image['uuid']);
+  }
+
+}

core/modules/layout_builder/src/Plugin/Block/InlineBlock.php

@@ -0,0 +1,299 @@
+<?php
+
+namespace Drupal\layout_builder\Plugin\Block;
+
+use Drupal\block_content\Access\RefinableDependentAccessInterface;
+use Drupal\block_content\Access\RefinableDependentAccessTrait;
+use Drupal\Component\Utility\NestedArray;
+use Drupal\Core\Access\AccessResult;
+use Drupal\Core\Block\BlockBase;
+use Drupal\Core\Entity\Entity\EntityFormDisplay;
+use Drupal\Core\Entity\EntityDisplayRepositoryInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Form\SubformStateInterface;
+use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
+use Drupal\Core\Session\AccountInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Defines an inline block plugin type.
+ *
+ * @Block(
+ *  id = "inline_block",
+ *  admin_label = @Translation("Inline block"),
+ *  category = @Translation("Inline blocks"),
+ *  deriver = "Drupal\layout_builder\Plugin\Derivative\InlineBlockDeriver",
+ * )
+ *
+ * @internal
+ *   Plugin classes are internal.
+ */
+class InlineBlock extends BlockBase implements ContainerFactoryPluginInterface, RefinableDependentAccessInterface {
+
+  use RefinableDependentAccessTrait;
+
+  /**
+   * The entity type manager.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
+   */
+  protected $entityTypeManager;
+
+  /**
+   * The block content entity.
+   *
+   * @var \Drupal\block_content\BlockContentInterface
+   */
+  protected $blockContent;
+
+  /**
+   * The entity display repository.
+   *
+   * @var \Drupal\Core\Entity\EntityDisplayRepositoryInterface
+   */
+  protected $entityDisplayRepository;
+
+  /**
+   * Whether a new block is being created.
+   *
+   * @var bool
+   */
+  protected $isNew = TRUE;
+
+  /**
+   * The current user.
+   *
+   * @var \Drupal\Core\Session\AccountInterface
+   */
+  protected $currentUser;
+
+  /**
+   * Constructs a new InlineBlock.
+   *
+   * @param array $configuration
+   *   A configuration array containing information about the plugin instance.
+   * @param string $plugin_id
+   *   The plugin ID for the plugin instance.
+   * @param mixed $plugin_definition
+   *   The plugin implementation definition.
+   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
+   *   The entity type manager service.
+   * @param \Drupal\Core\Entity\EntityDisplayRepositoryInterface $entity_display_repository
+   *   The entity display repository.
+   * @param \Drupal\Core\Session\AccountInterface $current_user
+   *   The current user.
+   */
+  public function __construct(array $configuration, $plugin_id, $plugin_definition, EntityTypeManagerInterface $entity_type_manager, EntityDisplayRepositoryInterface $entity_display_repository, AccountInterface $current_user) {
+    parent::__construct($configuration, $plugin_id, $plugin_definition);
+
+    $this->entityTypeManager = $entity_type_manager;
+    $this->entityDisplayRepository = $entity_display_repository;
+    $this->currentUser = $current_user;
+    if (!empty($this->configuration['block_revision_id']) || !empty($this->configuration['block_serialized'])) {
+      $this->isNew = FALSE;
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
+    return new static(
+      $configuration,
+      $plugin_id,
+      $plugin_definition,
+      $container->get('entity_type.manager'),
+      $container->get('entity_display.repository'),
+      $container->get('current_user')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function defaultConfiguration() {
+    return [
+      'view_mode' => 'full',
+      'block_id' => NULL,
+      'block_revision_id' => NULL,
+      'block_serialized' => NULL,
+    ];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function blockForm($form, FormStateInterface $form_state) {
+    $block = $this->getEntity();
+
+    // Add the entity form display in a process callback so that #parents can
+    // be successfully propagated to field widgets.
+    $form['block_form'] = [
+      '#type' => 'container',
+      '#process' => [[static::class, 'processBlockForm']],
+      '#block' => $block,
+      '#access' => $this->currentUser->hasPermission('create and edit custom blocks'),
+    ];
+
+    $options = $this->entityDisplayRepository->getViewModeOptionsByBundle('block_content', $block->bundle());
+
+    $form['view_mode'] = [
+      '#type' => 'select',
+      '#options' => $options,
+      '#title' => $this->t('View mode'),
+      '#description' => $this->t('The view mode in which to render the block.'),
+      '#default_value' => $this->configuration['view_mode'],
+      '#access' => count($options) > 1,
+    ];
+    return $form;
+  }
+
+  /**
+   * Process callback to insert a Content Block form.
+   *
+   * @param array $element
+   *   The containing element.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The form state.
+   *
+   * @return array
+   *   The containing element, with the Content Block form inserted.
+   */
+  public static function processBlockForm(array $element, FormStateInterface $form_state) {
+    /** @var \Drupal\block_content\BlockContentInterface $block */
+    $block = $element['#block'];
+    EntityFormDisplay::collectRenderDisplay($block, 'edit')->buildForm($block, $element, $form_state);
+    $element['revision_log']['#access'] = FALSE;
+    $element['info']['#access'] = FALSE;
+    return $element;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function blockValidate($form, FormStateInterface $form_state) {
+    $block_form = $form['block_form'];
+    /** @var \Drupal\block_content\BlockContentInterface $block */
+    $block = $block_form['#block'];
+    $form_display = EntityFormDisplay::collectRenderDisplay($block, 'edit');
+    $complete_form_state = $form_state instanceof SubformStateInterface ? $form_state->getCompleteFormState() : $form_state;
+    $form_display->extractFormValues($block, $block_form, $complete_form_state);
+    $form_display->validateFormValues($block, $block_form, $complete_form_state);
+    // @todo Remove when https://www.drupal.org/project/drupal/issues/2948549 is closed.
+    $form_state->setTemporaryValue('block_form_parents', $block_form['#parents']);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function blockSubmit($form, FormStateInterface $form_state) {
+    $this->configuration['view_mode'] = $form_state->getValue('view_mode');
+
+    // @todo Remove when https://www.drupal.org/project/drupal/issues/2948549 is closed.
+    $block_form = NestedArray::getValue($form, $form_state->getTemporaryValue('block_form_parents'));
+    /** @var \Drupal\block_content\BlockContentInterface $block */
+    $block = $block_form['#block'];
+    $form_display = EntityFormDisplay::collectRenderDisplay($block, 'edit');
+    $complete_form_state = $form_state instanceof SubformStateInterface ? $form_state->getCompleteFormState() : $form_state;
+    $form_display->extractFormValues($block, $block_form, $complete_form_state);
+    $block->setInfo($this->configuration['label']);
+    $this->configuration['block_serialized'] = serialize($block);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function blockAccess(AccountInterface $account) {
+    if ($entity = $this->getEntity()) {
+      return $entity->access('view', $account, TRUE);
+    }
+    return AccessResult::forbidden();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function build() {
+    $block = $this->getEntity();
+    return $this->entityTypeManager->getViewBuilder($block->getEntityTypeId())->view($block, $this->configuration['view_mode']);
+  }
+
+  /**
+   * Loads or creates the block content entity of the block.
+   *
+   * @return \Drupal\block_content\BlockContentInterface
+   *   The block content entity.
+   */
+  protected function getEntity() {
+    if (!isset($this->blockContent)) {
+      if (!empty($this->configuration['block_serialized'])) {
+        $this->blockContent = unserialize($this->configuration['block_serialized']);
+      }
+      elseif (!empty($this->configuration['block_revision_id'])) {
+        $entity = $this->entityTypeManager->getStorage('block_content')->loadRevision($this->configuration['block_revision_id']);
+        $this->blockContent = $entity;
+      }
+      else {
+        $this->blockContent = $this->entityTypeManager->getStorage('block_content')->create([
+          'type' => $this->getDerivativeId(),
+          'reusable' => FALSE,
+        ]);
+      }
+      if ($this->blockContent instanceof RefinableDependentAccessInterface && $dependee = $this->getAccessDependency()) {
+        $this->blockContent->setAccessDependency($dependee);
+      }
+    }
+    return $this->blockContent;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
+    $form = parent::buildConfigurationForm($form, $form_state);
+    if ($this->isNew) {
+      // If the Content Block is new then don't provide a default label.
+      unset($form['label']['#default_value']);
+    }
+    $form['label']['#description'] = $this->t('The title of the block as shown to the user.');
+    return $form;
+  }
+
+  /**
+   * Saves the block_content entity for this plugin.
+   *
+   * @param bool $new_revision
+   *   Whether to create new revision, if the block was modified.
+   * @param bool $duplicate_block
+   *   Whether to duplicate the "block_content" entity.
+   */
+  public function saveBlockContent($new_revision = FALSE, $duplicate_block = FALSE) {
+    /** @var \Drupal\block_content\BlockContentInterface $block */
+    $block = NULL;
+    if (!empty($this->configuration['block_serialized'])) {
+      $block = unserialize($this->configuration['block_serialized']);
+    }
+    if ($duplicate_block) {
+      if (empty($block) && !empty($this->configuration['block_revision_id'])) {
+        $block = $this->entityTypeManager->getStorage('block_content')->loadRevision($this->configuration['block_revision_id']);
+      }
+      if ($block) {
+        $block = $block->createDuplicate();
+      }
+    }
+
+    if ($block) {
+      // Since the content block is only set if it was unserialized, the flag
+      // will only effect blocks which were modified or serialized originally.
+      if ($new_revision) {
+        $block->setNewRevision();
+      }
+      $block->save();
+      $this->configuration['block_id'] = $block->id();
+      $this->configuration['block_revision_id'] = $block->getRevisionId();
+      $this->configuration['block_serialized'] = NULL;
+    }
+  }
+
+}

core/modules/layout_builder/src/Plugin/DataType/SectionData.php

@@ -0,0 +1,40 @@
+<?php
+
+namespace Drupal\layout_builder\Plugin\DataType;
+
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\Core\TypedData\Attribute\DataType;
+use Drupal\Core\TypedData\TypedData;
+use Drupal\layout_builder\Section;
+
+/**
+ * Provides a data type wrapping \Drupal\layout_builder\Section.
+ *
+ * @internal
+ *   Plugin classes are internal.
+ */
+#[DataType(
+  id: "layout_section",
+  label: new TranslatableMarkup("Layout Section"),
+  description: new TranslatableMarkup("A layout section"),
+)]
+class SectionData extends TypedData {
+
+  /**
+   * The section object.
+   *
+   * @var \Drupal\layout_builder\Section
+   */
+  protected $value;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setValue($value, $notify = TRUE) {
+    if ($value && !$value instanceof Section) {
+      throw new \InvalidArgumentException(sprintf('Value assigned to "%s" is not a valid section', $this->getName()));
+    }
+    parent::setValue($value, $notify);
+  }
+
+}

core/modules/layout_builder/src/Plugin/Derivative/ExtraFieldBlockDeriver.php

@@ -0,0 +1,167 @@
+<?php
+
+namespace Drupal\layout_builder\Plugin\Derivative;
+
+use Drupal\Component\Plugin\Derivative\DeriverBase;
+use Drupal\Component\Plugin\PluginBase;
+use Drupal\Core\Entity\EntityFieldManagerInterface;
+use Drupal\Core\Entity\EntityTypeBundleInfoInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Entity\FieldableEntityInterface;
+use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Core\Plugin\Context\EntityContextDefinition;
+use Drupal\Core\Plugin\Discovery\ContainerDeriverInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+use Drupal\Core\Entity\EntityTypeRepositoryInterface;
+
+/**
+ * Provides entity field block definitions for every field.
+ *
+ * @internal
+ *   Plugin derivers are internal.
+ */
+class ExtraFieldBlockDeriver extends DeriverBase implements ContainerDeriverInterface {
+
+  use StringTranslationTrait;
+
+  /**
+   * The entity field manager.
+   *
+   * @var \Drupal\Core\Entity\EntityFieldManagerInterface
+   */
+  protected $entityFieldManager;
+
+  /**
+   * The entity type manager.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
+   */
+  protected $entityTypeManager;
+
+  /**
+   * The entity type bundle info.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeBundleInfoInterface
+   */
+  protected $entityTypeBundleInfo;
+
+  /**
+   * The entity type repository.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeRepositoryInterface
+   */
+  protected $entityTypeRepository;
+
+  /**
+   * Constructs new FieldBlockDeriver.
+   *
+   * @param \Drupal\Core\Entity\EntityFieldManagerInterface $entity_field_manager
+   *   The entity field manager.
+   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
+   *   The entity type manager.
+   * @param \Drupal\Core\Entity\EntityTypeBundleInfoInterface $entity_type_bundle_info
+   *   The entity type bundle info.
+   * @param \Drupal\Core\Entity\EntityTypeRepositoryInterface $entity_type_repository
+   *   The entity type repository.
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface $moduleHandler
+   *   The module handler.
+   */
+  public function __construct(
+    EntityFieldManagerInterface $entity_field_manager,
+    EntityTypeManagerInterface $entity_type_manager,
+    EntityTypeBundleInfoInterface $entity_type_bundle_info,
+    EntityTypeRepositoryInterface $entity_type_repository,
+    protected ModuleHandlerInterface $moduleHandler,
+  ) {
+    $this->entityFieldManager = $entity_field_manager;
+    $this->entityTypeManager = $entity_type_manager;
+    $this->entityTypeBundleInfo = $entity_type_bundle_info;
+    $this->entityTypeRepository = $entity_type_repository;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, $base_plugin_id) {
+    return new static(
+      $container->get('entity_field.manager'),
+      $container->get('entity_type.manager'),
+      $container->get('entity_type.bundle.info'),
+      $container->get('entity_type.repository'),
+      $container->get('module_handler')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getDerivativeDefinitions($base_plugin_definition) {
+    $entity_type_labels = $this->entityTypeRepository->getEntityTypeLabels();
+    $enabled_bundle_ids = $this->bundleIdsWithLayoutBuilderDisplays();
+    $expose_all_fields = $this->moduleHandler->moduleExists('layout_builder_expose_all_field_blocks');
+    foreach ($this->entityTypeManager->getDefinitions() as $entity_type_id => $entity_type) {
+      // Only process fieldable entity types.
+      if (!$entity_type->entityClassImplements(FieldableEntityInterface::class)) {
+        continue;
+      }
+
+      // If not loading everything, skip entity types that aren't included.
+      if (!$expose_all_fields && !isset($enabled_bundle_ids[$entity_type_id])) {
+        continue;
+      }
+
+      $bundles = $this->entityTypeBundleInfo->getBundleInfo($entity_type_id);
+      foreach ($bundles as $bundle_id => $bundle) {
+        // If not loading everything, skip bundle types that aren't included.
+        if (!$expose_all_fields && !isset($enabled_bundle_ids[$entity_type_id][$bundle_id])) {
+          continue;
+        }
+
+        $extra_fields = $this->entityFieldManager->getExtraFields($entity_type_id, $bundle_id);
+        // Skip bundles without any extra fields.
+        if (empty($extra_fields['display'])) {
+          continue;
+        }
+
+        foreach ($extra_fields['display'] as $extra_field_id => $extra_field) {
+          $derivative = $base_plugin_definition;
+
+          $derivative['category'] = $this->t('@entity fields', ['@entity' => $entity_type_labels[$entity_type_id]]);
+
+          $derivative['admin_label'] = $extra_field['label'];
+
+          $context_definition = EntityContextDefinition::fromEntityType($entity_type)
+            ->addConstraint('Bundle', [$bundle_id]);
+          $derivative['context_definitions'] = [
+            'entity' => $context_definition,
+          ];
+
+          $derivative_id = $entity_type_id . PluginBase::DERIVATIVE_SEPARATOR . $bundle_id . PluginBase::DERIVATIVE_SEPARATOR . $extra_field_id;
+          $this->derivatives[$derivative_id] = $derivative;
+        }
+      }
+    }
+    return $this->derivatives;
+  }
+
+  /**
+   * Gets a list of entity type and bundle tuples that have layout builder enabled.
+   *
+   * @return array
+   *   A structured array with entity type as first key, bundle as second.
+   */
+  protected function bundleIdsWithLayoutBuilderDisplays(): array {
+    /** @var \Drupal\layout_builder\Entity\LayoutEntityDisplayInterface[] $displays */
+    $displays = $this->entityTypeManager->getStorage('entity_view_display')->loadByProperties([
+      'third_party_settings.layout_builder.enabled' => TRUE,
+    ]);
+    $layout_bundles = [];
+    foreach ($displays as $display) {
+      $bundle = $display->getTargetBundle();
+      $layout_bundles[$display->getTargetEntityTypeId()][$bundle] = $bundle;
+    }
+    return $layout_bundles;
+  }
+
+}

core/modules/layout_builder/src/Plugin/Derivative/FieldBlockDeriver.php

@@ -0,0 +1,206 @@
+<?php
+
+namespace Drupal\layout_builder\Plugin\Derivative;
+
+use Drupal\Component\Plugin\Derivative\DeriverBase;
+use Drupal\Component\Plugin\PluginBase;
+use Drupal\Core\Config\Entity\ConfigEntityStorageInterface;
+use Drupal\Core\Entity\EntityFieldManagerInterface;
+use Drupal\Core\Entity\EntityTypeRepositoryInterface;
+use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Core\Field\FieldConfigInterface;
+use Drupal\Core\Field\FieldTypePluginManagerInterface;
+use Drupal\Core\Field\FormatterPluginManager;
+use Drupal\Core\Logger\LoggerChannelTrait;
+use Drupal\Core\Plugin\Context\ContextDefinition;
+use Drupal\Core\Plugin\Context\EntityContextDefinition;
+use Drupal\Core\Plugin\Discovery\ContainerDeriverInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Provides entity field block definitions for every field.
+ *
+ * @internal
+ *   Plugin derivers are internal.
+ */
+class FieldBlockDeriver extends DeriverBase implements ContainerDeriverInterface {
+
+  use StringTranslationTrait;
+  use LoggerChannelTrait;
+
+  /**
+   * The entity type repository.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeRepositoryInterface
+   */
+  protected $entityTypeRepository;
+
+  /**
+   * The entity field manager.
+   *
+   * @var \Drupal\Core\Entity\EntityFieldManagerInterface
+   */
+  protected $entityFieldManager;
+
+  /**
+   * The field type manager.
+   *
+   * @var \Drupal\Core\Field\FieldTypePluginManagerInterface
+   */
+  protected $fieldTypeManager;
+
+  /**
+   * The formatter manager.
+   *
+   * @var \Drupal\Core\Field\FormatterPluginManager
+   */
+  protected $formatterManager;
+
+  /**
+   * Constructs new FieldBlockDeriver.
+   *
+   * @param \Drupal\Core\Entity\EntityTypeRepositoryInterface $entity_type_repository
+   *   The entity type repository.
+   * @param \Drupal\Core\Entity\EntityFieldManagerInterface $entity_field_manager
+   *   The entity field manager.
+   * @param \Drupal\Core\Field\FieldTypePluginManagerInterface $field_type_manager
+   *   The field type manager.
+   * @param \Drupal\Core\Field\FormatterPluginManager $formatter_manager
+   *   The formatter manager.
+   * @param \Drupal\Core\Config\Entity\ConfigEntityStorageInterface $entityViewDisplayStorage
+   *   The entity view display storage.
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface $moduleHandler
+   *   The module handler.
+   */
+  public function __construct(
+    EntityTypeRepositoryInterface $entity_type_repository,
+    EntityFieldManagerInterface $entity_field_manager,
+    FieldTypePluginManagerInterface $field_type_manager,
+    FormatterPluginManager $formatter_manager,
+    protected ConfigEntityStorageInterface $entityViewDisplayStorage,
+    protected ModuleHandlerInterface $moduleHandler,
+  ) {
+    $this->entityTypeRepository = $entity_type_repository;
+    $this->entityFieldManager = $entity_field_manager;
+    $this->fieldTypeManager = $field_type_manager;
+    $this->formatterManager = $formatter_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, $base_plugin_id) {
+    return new static(
+      $container->get('entity_type.repository'),
+      $container->get('entity_field.manager'),
+      $container->get('plugin.manager.field.field_type'),
+      $container->get('plugin.manager.field.formatter'),
+      $container->get('entity_type.manager')->getStorage('entity_view_display'),
+      $container->get('module_handler')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getDerivativeDefinitions($base_plugin_definition) {
+    $entity_type_labels = $this->entityTypeRepository->getEntityTypeLabels();
+    foreach ($this->getFieldMap() as $entity_type_id => $entity_field_map) {
+      foreach ($entity_field_map as $field_name => $field_info) {
+        // Skip fields without any formatters.
+        $options = $this->formatterManager->getOptions($field_info['type']);
+        if (empty($options)) {
+          continue;
+        }
+
+        foreach ($field_info['bundles'] as $bundle) {
+          $derivative = $base_plugin_definition;
+          $field_definitions = $this->entityFieldManager->getFieldDefinitions($entity_type_id, $bundle);
+          if (empty($field_definitions[$field_name])) {
+            $this->getLogger('field')->error('Field %field_name exists but is missing a corresponding field definition and may be misconfigured.', ['%field_name' => "$entity_type_id.$bundle.$field_name"]);
+            continue;
+          }
+          $field_definition = $field_definitions[$field_name];
+
+          // Store the default formatter on the definition.
+          $derivative['default_formatter'] = '';
+          $field_type_definition = $this->fieldTypeManager->getDefinition($field_info['type']);
+          if (isset($field_type_definition['default_formatter'])) {
+            $derivative['default_formatter'] = $field_type_definition['default_formatter'];
+          }
+
+          $derivative['category'] = $this->t('@entity fields', ['@entity' => $entity_type_labels[$entity_type_id]]);
+
+          $derivative['admin_label'] = $field_definition->getLabel();
+
+          // Add a dependency on the field if it is configurable.
+          if ($field_definition instanceof FieldConfigInterface) {
+            $derivative['config_dependencies'][$field_definition->getConfigDependencyKey()][] = $field_definition->getConfigDependencyName();
+          }
+          // For any field that is not display configurable, mark it as
+          // unavailable to place in the block UI.
+          $derivative['_block_ui_hidden'] = !$field_definition->isDisplayConfigurable('view');
+
+          $context_definition = EntityContextDefinition::fromEntityTypeId($entity_type_id)->setLabel($entity_type_labels[$entity_type_id]);
+          $context_definition->addConstraint('Bundle', [$bundle]);
+          $derivative['context_definitions'] = [
+            'entity' => $context_definition,
+            'view_mode' => new ContextDefinition('string'),
+          ];
+
+          $derivative_id = $entity_type_id . PluginBase::DERIVATIVE_SEPARATOR . $bundle . PluginBase::DERIVATIVE_SEPARATOR . $field_name;
+          $this->derivatives[$derivative_id] = $derivative;
+        }
+      }
+    }
+    return $this->derivatives;
+  }
+
+  /**
+   * Returns the entity field map for deriving block definitions.
+   *
+   * @return array
+   *   The entity field map.
+   *
+   * @see \Drupal\Core\Entity\EntityFieldManagerInterface::getFieldMap()
+   */
+  protected function getFieldMap(): array {
+    $field_map = $this->entityFieldManager->getFieldMap();
+
+    // If all fields are exposed as field blocks, just return the field map
+    // without any further processing.
+    if ($this->moduleHandler->moduleExists('layout_builder_expose_all_field_blocks')) {
+      return $field_map;
+    }
+
+    // Load all entity view displays which are using Layout Builder.
+    /** @var \Drupal\layout_builder\Entity\LayoutEntityDisplayInterface[] $displays */
+    $displays = $this->entityViewDisplayStorage->loadByProperties([
+      'third_party_settings.layout_builder.enabled' => TRUE,
+    ]);
+    $layout_bundles = [];
+    foreach ($displays as $display) {
+      $bundle = $display->getTargetBundle();
+      $layout_bundles[$display->getTargetEntityTypeId()][$bundle] = $bundle;
+    }
+
+    // Process $field_map, removing any entity types which are not using Layout
+    // Builder.
+    $field_map = array_intersect_key($field_map, $layout_bundles);
+
+    foreach ($field_map as $entity_type_id => $fields) {
+      foreach ($fields as $field_name => $field_info) {
+        $field_map[$entity_type_id][$field_name]['bundles'] = array_intersect($field_info['bundles'], $layout_bundles[$entity_type_id]);
+
+        // If no bundles are using Layout Builder, remove this field from the
+        // field map.
+        if (empty($field_info['bundles'])) {
+          unset($field_map[$entity_type_id][$field_name]);
+        }
+      }
+    }
+    return $field_map;
+  }
+
+}

core/modules/layout_builder/src/Plugin/Derivative/InlineBlockDeriver.php

@@ -0,0 +1,60 @@
+<?php
+
+namespace Drupal\layout_builder\Plugin\Derivative;
+
+use Drupal\Component\Plugin\Derivative\DeriverBase;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Plugin\Discovery\ContainerDeriverInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Provides inline block plugin definitions for all block types.
+ *
+ * @internal
+ *   Plugin derivers are internal.
+ */
+class InlineBlockDeriver extends DeriverBase implements ContainerDeriverInterface {
+
+  /**
+   * The entity type manager.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
+   */
+  protected $entityTypeManager;
+
+  /**
+   * Constructs a BlockContentDeriver object.
+   *
+   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
+   *   The entity type manager.
+   */
+  public function __construct(EntityTypeManagerInterface $entity_type_manager) {
+    $this->entityTypeManager = $entity_type_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, $base_plugin_id) {
+    return new static(
+      $container->get('entity_type.manager')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getDerivativeDefinitions($base_plugin_definition) {
+    $this->derivatives = [];
+    if ($this->entityTypeManager->hasDefinition('block_content_type')) {
+      $block_content_types = $this->entityTypeManager->getStorage('block_content_type')->loadMultiple();
+      foreach ($block_content_types as $id => $type) {
+        $this->derivatives[$id] = $base_plugin_definition;
+        $this->derivatives[$id]['admin_label'] = $type->label();
+        $this->derivatives[$id]['config_dependencies'][$type->getConfigDependencyKey()][] = $type->getConfigDependencyName();
+      }
+    }
+    return parent::getDerivativeDefinitions($base_plugin_definition);
+  }
+
+}

core/modules/layout_builder/src/Plugin/Derivative/LayoutBuilderLocalTaskDeriver.php

@@ -0,0 +1,75 @@
+<?php
+
+namespace Drupal\layout_builder\Plugin\Derivative;
+
+use Drupal\Component\Plugin\Derivative\DeriverBase;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Plugin\Discovery\ContainerDeriverInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\layout_builder\Plugin\SectionStorage\SectionStorageLocalTaskProviderInterface;
+use Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Provides local task definitions for the layout builder user interface.
+ *
+ * @todo Remove this in https://www.drupal.org/project/drupal/issues/2936655.
+ *
+ * @internal
+ *   Plugin derivers are internal.
+ */
+class LayoutBuilderLocalTaskDeriver extends DeriverBase implements ContainerDeriverInterface {
+
+  use StringTranslationTrait;
+
+  /**
+   * The entity type manager.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
+   */
+  protected $entityTypeManager;
+
+  /**
+   * The section storage manager.
+   *
+   * @var \Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface
+   */
+  protected $sectionStorageManager;
+
+  /**
+   * Constructs a new LayoutBuilderLocalTaskDeriver.
+   *
+   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
+   *   The entity type manager.
+   * @param \Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface $section_storage_manager
+   *   The section storage manager.
+   */
+  public function __construct(EntityTypeManagerInterface $entity_type_manager, SectionStorageManagerInterface $section_storage_manager) {
+    $this->entityTypeManager = $entity_type_manager;
+    $this->sectionStorageManager = $section_storage_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, $base_plugin_id) {
+    return new static(
+      $container->get('entity_type.manager'),
+      $container->get('plugin.manager.layout_builder.section_storage')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getDerivativeDefinitions($base_plugin_definition) {
+    foreach ($this->sectionStorageManager->getDefinitions() as $plugin_id => $definition) {
+      $section_storage = $this->sectionStorageManager->loadEmpty($plugin_id);
+      if ($section_storage instanceof SectionStorageLocalTaskProviderInterface) {
+        $this->derivatives += $section_storage->buildLocalTasks($base_plugin_definition);
+      }
+    }
+    return $this->derivatives;
+  }
+
+}

core/modules/layout_builder/src/Plugin/Field/FieldType/LayoutSectionItem.php

@@ -0,0 +1,96 @@
+<?php
+
+namespace Drupal\layout_builder\Plugin\Field\FieldType;
+
+use Drupal\Core\Field\Attribute\FieldType;
+use Drupal\Core\Field\FieldDefinitionInterface;
+use Drupal\Core\Field\FieldItemBase;
+use Drupal\Core\Field\FieldStorageDefinitionInterface;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\Core\TypedData\DataDefinition;
+use Drupal\layout_builder\Field\LayoutSectionItemList;
+use Drupal\layout_builder\Section;
+
+/**
+ * Plugin implementation of the 'layout_section' field type.
+ *
+ * @internal
+ *   Plugin classes are internal.
+ *
+ * @property \Drupal\layout_builder\Section $section
+ */
+#[FieldType(
+  id: "layout_section",
+  label: new TranslatableMarkup("Layout Section"),
+  description: new TranslatableMarkup("Layout Section"),
+  no_ui: TRUE,
+  list_class: LayoutSectionItemList::class,
+  cardinality: FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED
+)]
+class LayoutSectionItem extends FieldItemBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
+    $properties['section'] = DataDefinition::create('layout_section')
+      ->setLabel(new TranslatableMarkup('Layout Section'))
+      ->setRequired(FALSE);
+
+    return $properties;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function __get($name) {
+    // @todo \Drupal\Core\Field\FieldItemBase::__get() does not return default
+    //   values for un-instantiated properties. This will forcibly instantiate
+    //   all properties with the side-effect of a performance hit, resolve
+    //   properly in https://www.drupal.org/node/2413471.
+    $this->getProperties();
+
+    return parent::__get($name);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function mainPropertyName() {
+    return 'section';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function schema(FieldStorageDefinitionInterface $field_definition) {
+    $schema = [
+      'columns' => [
+        'section' => [
+          'type' => 'blob',
+          'size' => 'normal',
+          'serialize' => TRUE,
+        ],
+      ],
+    ];
+
+    return $schema;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function generateSampleValue(FieldDefinitionInterface $field_definition) {
+    // @todo Expand this in https://www.drupal.org/node/2912331.
+    $values['section'] = new Section('layout_onecol');
+    return $values;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function isEmpty() {
+    return empty($this->section);
+  }
+
+}

core/modules/layout_builder/src/Plugin/Field/FieldWidget/LayoutBuilderWidget.php

@@ -0,0 +1,78 @@
+<?php
+
+namespace Drupal\layout_builder\Plugin\Field\FieldWidget;
+
+use Drupal\Core\Field\Attribute\FieldWidget;
+use Drupal\Core\Field\FieldItemListInterface;
+use Drupal\Core\Field\WidgetBase;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+
+/**
+ * A widget to display the layout form.
+ *
+ * @internal
+ *   Plugin classes are internal.
+ */
+#[FieldWidget(
+  id: 'layout_builder_widget',
+  label: new TranslatableMarkup('Layout Builder Widget'),
+  description: new TranslatableMarkup('A field widget for Layout Builder.'),
+  field_types: ['layout_section'],
+  multiple_values: TRUE,
+)]
+class LayoutBuilderWidget extends WidgetBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
+    $element += [
+      '#type' => 'layout_builder',
+      '#section_storage' => $this->getSectionStorage($form_state),
+    ];
+    $element['#process'][] = [static::class, 'layoutBuilderElementGetKeys'];
+    return $element;
+  }
+
+  /**
+   * Form element #process callback.
+   *
+   * Save the layout builder element array parents as a property on the top form
+   * element so that they can be used to access the element within the whole
+   * render array later.
+   *
+   * @see \Drupal\layout_builder\Controller\LayoutBuilderHtmlEntityFormController
+   */
+  public static function layoutBuilderElementGetKeys(array $element, FormStateInterface $form_state, &$form) {
+    $form['#layout_builder_element_keys'] = $element['#array_parents'];
+    return $element;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function extractFormValues(FieldItemListInterface $items, array $form, FormStateInterface $form_state) {
+    // @todo This isn't resilient to being set twice, during validation and
+    //   save https://www.drupal.org/project/drupal/issues/2833682.
+    if (!$form_state->isValidationComplete()) {
+      return;
+    }
+
+    $items->setValue($this->getSectionStorage($form_state)->getSections());
+  }
+
+  /**
+   * Gets the section storage.
+   *
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The form state.
+   *
+   * @return \Drupal\layout_builder\SectionStorageInterface
+   *   The section storage loaded from the tempstore.
+   */
+  private function getSectionStorage(FormStateInterface $form_state) {
+    return $form_state->getFormObject()->getSectionStorage();
+  }
+
+}

core/modules/layout_builder/src/Plugin/Layout/BlankLayout.php

@@ -0,0 +1,33 @@
+<?php
+
+namespace Drupal\layout_builder\Plugin\Layout;
+
+use Drupal\Core\Layout\Attribute\Layout;
+use Drupal\Core\Layout\LayoutDefault;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+
+/**
+ * Provides a layout plugin that produces no output.
+ *
+ * @see \Drupal\layout_builder\Field\LayoutSectionItemList::removeSection()
+ * @see \Drupal\layout_builder\SectionListTrait::addBlankSection()
+ * @see \Drupal\layout_builder\SectionListTrait::hasBlankSection()
+ *
+ * @internal
+ *   This layout plugin is intended for internal use by Layout Builder only.
+ */
+#[Layout(
+  id: 'layout_builder_blank',
+  label: new TranslatableMarkup('Blank'),
+)]
+class BlankLayout extends LayoutDefault {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function build(array $regions) {
+    // Return no output.
+    return [];
+  }
+
+}

core/modules/layout_builder/src/Plugin/Layout/MultiWidthLayoutBase.php

@@ -0,0 +1,83 @@
+<?php
+
+namespace Drupal\layout_builder\Plugin\Layout;
+
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Layout\LayoutDefault;
+use Drupal\Core\Plugin\PluginFormInterface;
+
+/**
+ * Base class of layouts with configurable widths.
+ */
+abstract class MultiWidthLayoutBase extends LayoutDefault implements PluginFormInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function defaultConfiguration() {
+    $configuration = parent::defaultConfiguration();
+    return $configuration + [
+      'column_widths' => $this->getDefaultWidth(),
+    ];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
+    $form['column_widths'] = [
+      '#type' => 'select',
+      '#title' => $this->t('Column widths'),
+      '#default_value' => $this->configuration['column_widths'],
+      '#options' => $this->getWidthOptions(),
+      '#description' => $this->t('Choose the column widths for this layout.'),
+    ];
+    return parent::buildConfigurationForm($form, $form_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitConfigurationForm(array &$form, FormStateInterface $form_state) {
+    parent::submitConfigurationForm($form, $form_state);
+    $this->configuration['column_widths'] = $form_state->getValue('column_widths');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function build(array $regions) {
+    $build = parent::build($regions);
+    $build['#attributes']['class'] = [
+      'layout',
+      $this->getPluginDefinition()->getTemplate(),
+      $this->getPluginDefinition()->getTemplate() . '--' . $this->configuration['column_widths'],
+    ];
+    return $build;
+  }
+
+  /**
+   * Gets the width options for the configuration form.
+   *
+   * The first option will be used as the default 'column_widths' configuration
+   * value.
+   *
+   * @return string[]
+   *   The width options array where the keys are strings that will be added to
+   *   the CSS classes and the values are the human readable labels.
+   */
+  abstract protected function getWidthOptions();
+
+  /**
+   * Provides a default value for the width options.
+   *
+   * @return string
+   *   A key from the array returned by ::getWidthOptions().
+   */
+  protected function getDefaultWidth() {
+    // Return the first available key from the list of options.
+    $width_classes = array_keys($this->getWidthOptions());
+    return array_shift($width_classes);
+  }
+
+}

core/modules/layout_builder/src/Plugin/Layout/ThreeColumnLayout.php

@@ -0,0 +1,32 @@
+<?php
+
+namespace Drupal\layout_builder\Plugin\Layout;
+
+/**
+ * Configurable three column layout plugin class.
+ *
+ * @internal
+ *   Plugin classes are internal.
+ */
+class ThreeColumnLayout extends MultiWidthLayoutBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getWidthOptions() {
+    return [
+      '25-50-25' => '25%/50%/25%',
+      '33-34-33' => '33%/34%/33%',
+      '25-25-50' => '25%/25%/50%',
+      '50-25-25' => '50%/25%/25%',
+    ];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getDefaultWidth() {
+    return '33-34-33';
+  }
+
+}

core/modules/layout_builder/src/Plugin/Layout/TwoColumnLayout.php

@@ -0,0 +1,33 @@
+<?php
+
+namespace Drupal\layout_builder\Plugin\Layout;
+
+/**
+ * Configurable two column layout plugin class.
+ *
+ * @internal
+ *   Plugin classes are internal.
+ */
+class TwoColumnLayout extends MultiWidthLayoutBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getWidthOptions() {
+    return [
+      '50-50' => '50%/50%',
+      '33-67' => '33%/67%',
+      '67-33' => '67%/33%',
+      '25-75' => '25%/75%',
+      '75-25' => '75%/25%',
+    ];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getDefaultWidth() {
+    return '50-50';
+  }
+
+}

core/modules/layout_builder/src/Plugin/SectionStorage/DefaultsSectionStorage.php

@@ -0,0 +1,416 @@
+<?php
+
+namespace Drupal\layout_builder\Plugin\SectionStorage;
+
+use Drupal\Component\Plugin\Context\ContextInterface as ComponentContextInterface;
+use Drupal\Component\Utility\NestedArray;
+use Drupal\Core\Access\AccessResult;
+use Drupal\Core\Cache\RefinableCacheableDependencyInterface;
+use Drupal\Core\Entity\EntityTypeBundleInfoInterface;
+use Drupal\Core\Entity\EntityTypeInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Entity\FieldableEntityInterface;
+use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
+use Drupal\Core\Plugin\Context\ContextDefinition;
+use Drupal\Core\Plugin\Context\EntityContext;
+use Drupal\Core\Plugin\Context\EntityContextDefinition;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\Core\Url;
+use Drupal\layout_builder\Attribute\SectionStorage;
+use Drupal\layout_builder\DefaultsSectionStorageInterface;
+use Drupal\layout_builder\Entity\SampleEntityGeneratorInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+use Symfony\Component\Routing\RouteCollection;
+
+/**
+ * Defines the 'defaults' section storage type.
+ *
+ * DefaultsSectionStorage uses a positive weight because:
+ * - It must be picked after
+ *   \Drupal\layout_builder\Plugin\SectionStorage\OverridesSectionStorage.
+ * - The default weight is 0, so other custom implementations will also take
+ *   precedence unless otherwise specified.
+ *
+ * @internal
+ *   Plugin classes are internal.
+ */
+#[SectionStorage(id: "defaults", weight: 20, context_definitions: [
+  "display" => new EntityContextDefinition(
+    data_type: "entity_view_display",
+    label: new TranslatableMarkup("Entity view display"),
+  ),
+  'view_mode' => new ContextDefinition(
+    data_type: 'string',
+    label: new TranslatableMarkup("View mode"),
+    default_value: "default",
+  ),
+])]
+class DefaultsSectionStorage extends SectionStorageBase implements ContainerFactoryPluginInterface, DefaultsSectionStorageInterface {
+
+  /**
+   * The entity type manager.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
+   */
+  protected $entityTypeManager;
+
+  /**
+   * The entity type bundle info.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeBundleInfoInterface
+   */
+  protected $entityTypeBundleInfo;
+
+  /**
+   * The sample entity generator.
+   *
+   * @var \Drupal\layout_builder\Entity\SampleEntityGeneratorInterface
+   */
+  protected $sampleEntityGenerator;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function __construct(array $configuration, $plugin_id, $plugin_definition, EntityTypeManagerInterface $entity_type_manager, EntityTypeBundleInfoInterface $entity_type_bundle_info, SampleEntityGeneratorInterface $sample_entity_generator) {
+    parent::__construct($configuration, $plugin_id, $plugin_definition);
+
+    $this->entityTypeManager = $entity_type_manager;
+    $this->entityTypeBundleInfo = $entity_type_bundle_info;
+    $this->sampleEntityGenerator = $sample_entity_generator;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
+    return new static(
+      $configuration,
+      $plugin_id,
+      $plugin_definition,
+      $container->get('entity_type.manager'),
+      $container->get('entity_type.bundle.info'),
+      $container->get('layout_builder.sample_entity_generator')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getSectionList() {
+    return $this->getContextValue('display');
+  }
+
+  /**
+   * Gets the entity storing the defaults.
+   *
+   * @return \Drupal\layout_builder\Entity\LayoutEntityDisplayInterface
+   *   The entity storing the defaults.
+   */
+  protected function getDisplay() {
+    return $this->getSectionList();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getStorageId() {
+    return $this->getDisplay()->id();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getRedirectUrl() {
+    return Url::fromRoute("entity.entity_view_display.{$this->getDisplay()->getTargetEntityTypeId()}.view_mode", $this->getRouteParameters());
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getLayoutBuilderUrl($rel = 'view') {
+    return Url::fromRoute("layout_builder.{$this->getStorageType()}.{$this->getDisplay()->getTargetEntityTypeId()}.$rel", $this->getRouteParameters());
+  }
+
+  /**
+   * Provides the route parameters needed to generate a URL for this object.
+   *
+   * @return mixed[]
+   *   An associative array of parameter names and values.
+   */
+  protected function getRouteParameters() {
+    $display = $this->getDisplay();
+    $entity_type = $this->entityTypeManager->getDefinition($display->getTargetEntityTypeId());
+    $bundle_parameter_key = $entity_type->getBundleEntityType() ?: 'bundle';
+    return [
+      $bundle_parameter_key => $display->getTargetBundle(),
+      'view_mode_name' => $display->getMode(),
+    ];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildRoutes(RouteCollection $collection) {
+    if (!\Drupal::moduleHandler()->moduleExists('field_ui')) {
+      return;
+    }
+
+    foreach ($this->getEntityTypes() as $entity_type_id => $entity_type) {
+      // Try to get the route from the current collection.
+      if (!$entity_route = $collection->get($entity_type->get('field_ui_base_route'))) {
+        continue;
+      }
+
+      $path = $entity_route->getPath() . '/display/{view_mode_name}/layout';
+
+      $defaults = [];
+      $defaults['entity_type_id'] = $entity_type_id;
+      // If the entity type has no bundles and it doesn't use {bundle} in its
+      // admin path, use the entity type.
+      if (!str_contains($path, '{bundle}')) {
+        if (!$entity_type->hasKey('bundle')) {
+          $defaults['bundle'] = $entity_type_id;
+        }
+        else {
+          $defaults['bundle_key'] = $entity_type->getBundleEntityType();
+        }
+      }
+
+      $requirements = [];
+      $requirements['_field_ui_view_mode_access'] = 'administer ' . $entity_type_id . ' display';
+
+      $options = $entity_route->getOptions();
+      $options['_admin_route'] = FALSE;
+
+      $this->buildLayoutRoutes($collection, $this->getPluginDefinition(), $path, $defaults, $requirements, $options, $entity_type_id, 'entity_view_display');
+
+      // Set field_ui.route_enhancer to run on the manage layout form.
+      if (isset($defaults['bundle_key'])) {
+        $collection->get("layout_builder.defaults.$entity_type_id.view")
+          ->setOption('_field_ui', TRUE)
+          ->setDefault('bundle', '');
+      }
+
+      $route_names = [
+        "entity.entity_view_display.{$entity_type_id}.default",
+        "entity.entity_view_display.{$entity_type_id}.view_mode",
+      ];
+      foreach ($route_names as $route_name) {
+        if (!$route = $collection->get($route_name)) {
+          continue;
+        }
+
+        $route->addDefaults([
+          'section_storage_type' => $this->getStorageType(),
+          'section_storage' => '',
+        ] + $defaults);
+        $parameters['section_storage']['layout_builder_tempstore'] = TRUE;
+        $parameters = NestedArray::mergeDeep($parameters, $route->getOption('parameters') ?: []);
+        $route->setOption('parameters', $parameters);
+      }
+    }
+  }
+
+  /**
+   * Returns an array of relevant entity types.
+   *
+   * @return \Drupal\Core\Entity\EntityTypeInterface[]
+   *   An array of entity types.
+   */
+  protected function getEntityTypes() {
+    return array_filter($this->entityTypeManager->getDefinitions(), function (EntityTypeInterface $entity_type) {
+      return $entity_type->entityClassImplements(FieldableEntityInterface::class) && $entity_type->hasHandlerClass('form', 'layout_builder') && $entity_type->hasViewBuilderClass() && $entity_type->get('field_ui_base_route');
+    });
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getContextsDuringPreview() {
+    $contexts = parent::getContextsDuringPreview();
+
+    // During preview add a sample entity for the target entity type and bundle.
+    $display = $this->getDisplay();
+    $entity = $this->sampleEntityGenerator->get($display->getTargetEntityTypeId(), $display->getTargetBundle());
+
+    $contexts['layout_builder.entity'] = EntityContext::fromEntity($entity);
+    return $contexts;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function deriveContextsFromRoute($value, $definition, $name, array $defaults) {
+    $contexts = [];
+
+    if ($entity = $this->extractEntityFromRoute($value, $defaults)) {
+      $contexts['display'] = EntityContext::fromEntity($entity);
+    }
+    return $contexts;
+  }
+
+  /**
+   * Extracts an entity from the route values.
+   *
+   * @param mixed $value
+   *   The raw value from the route.
+   * @param array $defaults
+   *   The route defaults array.
+   *
+   * @return \Drupal\Core\Entity\EntityInterface|null
+   *   The entity for the route, or NULL if none exist.
+   *
+   * @see \Drupal\layout_builder\SectionStorageInterface::deriveContextsFromRoute()
+   * @see \Drupal\Core\ParamConverter\ParamConverterInterface::convert()
+   */
+  private function extractEntityFromRoute($value, array $defaults) {
+    // If a bundle is not provided but a value corresponding to the bundle key
+    // is, use that for the bundle value.
+    if (empty($defaults['bundle']) && isset($defaults['bundle_key']) && !empty($defaults[$defaults['bundle_key']])) {
+      $defaults['bundle'] = $defaults[$defaults['bundle_key']];
+    }
+
+    if (is_string($value) && str_contains($value, '.')) {
+      [$entity_type_id, $bundle, $view_mode] = explode('.', $value, 3);
+    }
+    elseif (!empty($defaults['entity_type_id']) && !empty($defaults['bundle']) && !empty($defaults['view_mode_name'])) {
+      $entity_type_id = $defaults['entity_type_id'];
+      $bundle = $defaults['bundle'];
+      $view_mode = $defaults['view_mode_name'];
+      $value = "$entity_type_id.$bundle.$view_mode";
+    }
+    else {
+      return NULL;
+    }
+
+    $storage = $this->entityTypeManager->getStorage('entity_view_display');
+    // If the display does not exist, create a new one.
+    if (!$display = $storage->load($value)) {
+      $display = $storage->create([
+        'targetEntityType' => $entity_type_id,
+        'bundle' => $bundle,
+        'mode' => $view_mode,
+        'status' => TRUE,
+      ]);
+    }
+    return $display;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function label() {
+    return $this->getDisplay()->label();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function save() {
+    return $this->getDisplay()->save();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function isOverridable() {
+    return $this->getDisplay()->isOverridable();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setOverridable($overridable = TRUE) {
+    $this->getDisplay()->setOverridable($overridable);
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setThirdPartySetting($module, $key, $value) {
+    $this->getDisplay()->setThirdPartySetting($module, $key, $value);
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function isLayoutBuilderEnabled() {
+    return $this->getDisplay()->isLayoutBuilderEnabled();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function enableLayoutBuilder() {
+    $this->getDisplay()->enableLayoutBuilder();
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function disableLayoutBuilder() {
+    $this->getDisplay()->disableLayoutBuilder();
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getThirdPartySetting($module, $key, $default = NULL) {
+    return $this->getDisplay()->getThirdPartySetting($module, $key, $default);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getThirdPartySettings($module) {
+    return $this->getDisplay()->getThirdPartySettings($module);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function unsetThirdPartySetting($module, $key) {
+    $this->getDisplay()->unsetThirdPartySetting($module, $key);
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getThirdPartyProviders() {
+    return $this->getDisplay()->getThirdPartyProviders();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function access($operation, ?AccountInterface $account = NULL, $return_as_object = FALSE) {
+    $result = AccessResult::allowedIf($this->isLayoutBuilderEnabled())->addCacheableDependency($this);
+    return $return_as_object ? $result : $result->isAllowed();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function isApplicable(RefinableCacheableDependencyInterface $cacheability) {
+    $cacheability->addCacheableDependency($this);
+    return $this->isLayoutBuilderEnabled();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setContext($name, ComponentContextInterface $context) {
+    // Set the view mode context based on the display context.
+    if ($name === 'display') {
+      $this->setContextValue('view_mode', $context->getContextValue()->getMode());
+    }
+    parent::setContext($name, $context);
+  }
+
+}

core/modules/layout_builder/src/Plugin/SectionStorage/OverridesSectionStorage.php

@@ -0,0 +1,417 @@
+<?php
+
+namespace Drupal\layout_builder\Plugin\SectionStorage;
+
+use Drupal\Core\Access\AccessResult;
+use Drupal\Core\Cache\RefinableCacheableDependencyInterface;
+use Drupal\Core\Entity\EntityFieldManagerInterface;
+use Drupal\Core\Entity\EntityRepositoryInterface;
+use Drupal\Core\Entity\EntityTypeInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Entity\FieldableEntityInterface;
+use Drupal\Core\Entity\TranslatableInterface;
+use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
+use Drupal\Core\Plugin\Context\Context;
+use Drupal\Core\Plugin\Context\ContextDefinition;
+use Drupal\Core\Plugin\Context\EntityContext;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\Core\Url;
+use Drupal\layout_builder\Attribute\SectionStorage;
+use Drupal\layout_builder\Entity\LayoutBuilderEntityViewDisplay;
+use Drupal\layout_builder\OverridesSectionStorageInterface;
+use Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+use Symfony\Component\Routing\RouteCollection;
+
+/**
+ * Defines the 'overrides' section storage type.
+ *
+ * OverridesSectionStorage uses a negative weight because:
+ * - It must be picked before
+ *   \Drupal\layout_builder\Plugin\SectionStorage\DefaultsSectionStorage.
+ * - The default weight is 0, so custom implementations will not take
+ *   precedence unless otherwise specified.
+ *
+ * @internal
+ *   Plugin classes are internal.
+ */
+#[SectionStorage(
+  id: "overrides",
+  weight: -20,
+  context_definitions: [
+    'entity' => new ContextDefinition(
+      data_type: 'entity',
+      label: new TranslatableMarkup("Entity"),
+      constraints: [
+        "EntityHasField" => OverridesSectionStorage::FIELD_NAME,
+      ],
+    ),
+    'view_mode' => new ContextDefinition(
+      data_type: 'string',
+      label: new TranslatableMarkup("View mode"),
+      default_value: "default",
+    ),
+  ],
+  handles_permission_check: TRUE,
+)]
+class OverridesSectionStorage extends SectionStorageBase implements ContainerFactoryPluginInterface, OverridesSectionStorageInterface, SectionStorageLocalTaskProviderInterface {
+
+  /**
+   * The field name used by this storage.
+   *
+   * @var string
+   */
+  const FIELD_NAME = 'layout_builder__layout';
+
+  /**
+   * The entity type manager.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
+   */
+  protected $entityTypeManager;
+
+  /**
+   * The entity field manager.
+   *
+   * @var \Drupal\Core\Entity\EntityFieldManagerInterface
+   */
+  protected $entityFieldManager;
+
+  /**
+   * The section storage manager.
+   *
+   * @var \Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface
+   */
+  protected $sectionStorageManager;
+
+  /**
+   * The entity repository.
+   *
+   * @var \Drupal\Core\Entity\EntityRepositoryInterface
+   */
+  protected $entityRepository;
+
+  /**
+   * The current user.
+   *
+   * @var \Drupal\Core\Session\AccountInterface
+   */
+  protected $currentUser;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function __construct(array $configuration, $plugin_id, $plugin_definition, EntityTypeManagerInterface $entity_type_manager, EntityFieldManagerInterface $entity_field_manager, SectionStorageManagerInterface $section_storage_manager, EntityRepositoryInterface $entity_repository, AccountInterface $current_user) {
+    parent::__construct($configuration, $plugin_id, $plugin_definition);
+
+    $this->entityTypeManager = $entity_type_manager;
+    $this->entityFieldManager = $entity_field_manager;
+    $this->sectionStorageManager = $section_storage_manager;
+    $this->entityRepository = $entity_repository;
+    $this->currentUser = $current_user;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
+    return new static(
+      $configuration,
+      $plugin_id,
+      $plugin_definition,
+      $container->get('entity_type.manager'),
+      $container->get('entity_field.manager'),
+      $container->get('plugin.manager.layout_builder.section_storage'),
+      $container->get('entity.repository'),
+      $container->get('current_user')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getSectionList() {
+    return $this->getEntity()->get(static::FIELD_NAME);
+  }
+
+  /**
+   * Gets the entity storing the overrides.
+   *
+   * @return \Drupal\Core\Entity\FieldableEntityInterface
+   *   The entity storing the overrides.
+   */
+  protected function getEntity() {
+    return $this->getContextValue('entity');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getStorageId() {
+    $entity = $this->getEntity();
+    return $entity->getEntityTypeId() . '.' . $entity->id();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getTempstoreKey() {
+    $key = parent::getTempstoreKey();
+    $key .= '.' . $this->getContextValue('view_mode');
+
+    $entity = $this->getEntity();
+    // @todo Allow entities to provide this contextual information in
+    //   https://www.drupal.org/project/drupal/issues/3026957.
+    if ($entity instanceof TranslatableInterface) {
+      $key .= '.' . $entity->language()->getId();
+    }
+    return $key;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function deriveContextsFromRoute($value, $definition, $name, array $defaults) {
+    $contexts = [];
+
+    if ($entity = $this->extractEntityFromRoute($value, $defaults)) {
+      $contexts['entity'] = EntityContext::fromEntity($entity);
+      // @todo Expand to work for all view modes in
+      //   https://www.drupal.org/node/2907413.
+      $view_mode = 'full';
+      // Retrieve the actual view mode from the returned view display as the
+      // requested view mode may not exist and a fallback will be used.
+      $view_mode = LayoutBuilderEntityViewDisplay::collectRenderDisplay($entity, $view_mode)->getMode();
+      $contexts['view_mode'] = new Context(new ContextDefinition('string'), $view_mode);
+    }
+    return $contexts;
+  }
+
+  /**
+   * Extracts an entity from the route values.
+   *
+   * @param mixed $value
+   *   The raw value from the route.
+   * @param array $defaults
+   *   The route defaults array.
+   *
+   * @return \Drupal\Core\Entity\EntityInterface|null
+   *   The entity for the route, or NULL if none exist. The entity is not
+   *   guaranteed to be fieldable, or contain the necessary field for this
+   *   section storage plugin.
+   *
+   * @see \Drupal\layout_builder\SectionStorageInterface::deriveContextsFromRoute()
+   * @see \Drupal\Core\ParamConverter\ParamConverterInterface::convert()
+   */
+  private function extractEntityFromRoute($value, array $defaults) {
+    if (str_contains($value, '.')) {
+      [$entity_type_id, $entity_id] = explode('.', $value, 2);
+    }
+    elseif (isset($defaults['entity_type_id']) && !empty($defaults[$defaults['entity_type_id']])) {
+      $entity_type_id = $defaults['entity_type_id'];
+      $entity_id = $defaults[$entity_type_id];
+    }
+    else {
+      return NULL;
+    }
+
+    $entity = $this->entityRepository->getActive($entity_type_id, $entity_id);
+    return ($entity instanceof FieldableEntityInterface) ? $entity : NULL;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildRoutes(RouteCollection $collection) {
+    foreach ($this->getEntityTypes() as $entity_type_id => $entity_type) {
+      // If the canonical route does not exist, do not provide any Layout
+      // Builder UI routes for this entity type.
+      if (!$collection->get("entity.$entity_type_id.canonical")) {
+        continue;
+      }
+
+      $defaults = [];
+      $defaults['entity_type_id'] = $entity_type_id;
+
+      // Retrieve the requirements from the canonical route.
+      $requirements = $collection->get("entity.$entity_type_id.canonical")->getRequirements();
+
+      $options = [];
+      // Ensure that upcasting is run in the correct order.
+      $options['parameters']['section_storage'] = [];
+      $options['parameters'][$entity_type_id]['type'] = 'entity:' . $entity_type_id;
+      $options['_admin_route'] = FALSE;
+
+      $template = $entity_type->getLinkTemplate('canonical') . '/layout';
+      $this->buildLayoutRoutes($collection, $this->getPluginDefinition(), $template, $defaults, $requirements, $options, $entity_type_id, $entity_type_id);
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildLocalTasks($base_plugin_definition) {
+    $local_tasks = [];
+    foreach ($this->getEntityTypes() as $entity_type_id => $entity_type) {
+      $local_tasks["layout_builder.overrides.$entity_type_id.view"] = $base_plugin_definition + [
+        'route_name' => "layout_builder.overrides.$entity_type_id.view",
+        'weight' => 15,
+        'title' => $this->t('Layout'),
+        'base_route' => "entity.$entity_type_id.canonical",
+        'cache_contexts' => ['layout_builder_is_active:' . $entity_type_id],
+      ];
+    }
+    return $local_tasks;
+  }
+
+  /**
+   * Determines if this entity type's ID is stored as an integer.
+   *
+   * @param \Drupal\Core\Entity\EntityTypeInterface $entity_type
+   *   An entity type.
+   *
+   * @return bool
+   *   TRUE if this entity type's ID key is always an integer, FALSE otherwise.
+   */
+  protected function hasIntegerId(EntityTypeInterface $entity_type) {
+    $field_storage_definitions = $this->entityFieldManager->getFieldStorageDefinitions($entity_type->id());
+    return $field_storage_definitions[$entity_type->getKey('id')]->getType() === 'integer';
+  }
+
+  /**
+   * Returns an array of relevant entity types.
+   *
+   * @return \Drupal\Core\Entity\EntityTypeInterface[]
+   *   An array of entity types.
+   */
+  protected function getEntityTypes() {
+    return array_filter($this->entityTypeManager->getDefinitions(), function (EntityTypeInterface $entity_type) {
+      return $entity_type->entityClassImplements(FieldableEntityInterface::class) && $entity_type->hasHandlerClass('form', 'layout_builder') && $entity_type->hasViewBuilderClass() && $entity_type->hasLinkTemplate('canonical');
+    });
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getDefaultSectionStorage() {
+    $display = LayoutBuilderEntityViewDisplay::collectRenderDisplay($this->getEntity(), $this->getContextValue('view_mode'));
+    return $this->sectionStorageManager->load('defaults', ['display' => EntityContext::fromEntity($display)]);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getRedirectUrl() {
+    return $this->getEntity()->toUrl('canonical');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getLayoutBuilderUrl($rel = 'view') {
+    $entity = $this->getEntity();
+    $route_parameters[$entity->getEntityTypeId()] = $entity->id();
+    return Url::fromRoute("layout_builder.{$this->getStorageType()}.{$this->getEntity()->getEntityTypeId()}.$rel", $route_parameters);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getContextsDuringPreview() {
+    $contexts = parent::getContextsDuringPreview();
+
+    if (isset($contexts['entity'])) {
+      $contexts['layout_builder.entity'] = $contexts['entity'];
+      unset($contexts['entity']);
+    }
+    return $contexts;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function label() {
+    return $this->getEntity()->label();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function save() {
+    return $this->getEntity()->save();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function access($operation, ?AccountInterface $account = NULL, $return_as_object = FALSE) {
+    if ($account === NULL) {
+      $account = $this->currentUser;
+    }
+
+    $entity = $this->getEntity();
+
+    // Create an access result that will allow access to the layout if one of
+    // these conditions applies:
+    // 1. The user can configure any layouts.
+    $any_access = AccessResult::allowedIfHasPermission($account, 'configure any layout');
+    // 2. The user can configure layouts on all items of the bundle type.
+    $bundle_access = AccessResult::allowedIfHasPermission($account, "configure all {$entity->bundle()} {$entity->getEntityTypeId()} layout overrides");
+    // 3. The user can configure layouts items of this bundle type they can edit
+    //    AND the user has access to edit this entity.
+    $edit_only_bundle_access = AccessResult::allowedIfHasPermission($account, "configure editable {$entity->bundle()} {$entity->getEntityTypeId()} layout overrides");
+    $edit_only_bundle_access = $edit_only_bundle_access->andIf($entity->access('update', $account, TRUE));
+
+    $result = $any_access
+      ->orIf($bundle_access)
+      ->orIf($edit_only_bundle_access);
+
+    // Access also depends on the default being enabled.
+    $result = $result->andIf($this->getDefaultSectionStorage()->access($operation, $account, TRUE));
+    // Access also depends on the default layout being overridable.
+    $result = $result->andIf(AccessResult::allowedIf($this->getDefaultSectionStorage()->isOverridable())->addCacheableDependency($this->getDefaultSectionStorage()));
+    $result = $this->handleTranslationAccess($result, $operation, $account);
+    return $return_as_object ? $result : $result->isAllowed();
+  }
+
+  /**
+   * Handles access checks related to translations.
+   *
+   * @param \Drupal\Core\Access\AccessResult $result
+   *   The access result.
+   * @param string $operation
+   *   The operation to be performed.
+   * @param \Drupal\Core\Session\AccountInterface $account
+   *   The user for which to check access.
+   *
+   * @return \Drupal\Core\Access\AccessResultInterface
+   *   The access result.
+   */
+  protected function handleTranslationAccess(AccessResult $result, $operation, AccountInterface $account) {
+    $entity = $this->getEntity();
+    // Access is always denied on non-default translations.
+    return $result->andIf(AccessResult::allowedIf(!($entity instanceof TranslatableInterface && !$entity->isDefaultTranslation())))->addCacheableDependency($entity);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function isApplicable(RefinableCacheableDependencyInterface $cacheability) {
+    $default_section_storage = $this->getDefaultSectionStorage();
+    $cacheability->addCacheableDependency($default_section_storage)->addCacheableDependency($this);
+    // Check that overrides are enabled and have at least one section.
+    return $default_section_storage->isOverridable() && $this->isOverridden();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function isOverridden() {
+    // If there are any sections at all, including a blank one, this section
+    // storage has been overridden. Do not use count() as it does not include
+    // blank sections.
+    return !empty($this->getSections());
+  }
+
+}

core/modules/layout_builder/src/Plugin/SectionStorage/SectionStorageBase.php

@@ -0,0 +1,115 @@
+<?php
+
+namespace Drupal\layout_builder\Plugin\SectionStorage;
+
+use Drupal\Core\Cache\CacheableDependencyInterface;
+use Drupal\Core\Plugin\Context\Context;
+use Drupal\Core\Plugin\Context\ContextDefinition;
+use Drupal\Core\Plugin\ContextAwarePluginTrait;
+use Drupal\Core\Plugin\PluginBase;
+use Drupal\layout_builder\Routing\LayoutBuilderRoutesTrait;
+use Drupal\layout_builder\Section;
+use Drupal\layout_builder\SectionStorageInterface;
+use Drupal\layout_builder\TempStoreIdentifierInterface;
+
+/**
+ * Provides a base class for Section Storage types.
+ */
+abstract class SectionStorageBase extends PluginBase implements SectionStorageInterface, TempStoreIdentifierInterface, CacheableDependencyInterface {
+
+  use ContextAwarePluginTrait;
+  use LayoutBuilderRoutesTrait;
+
+  /**
+   * Gets the section list.
+   *
+   * @return \Drupal\layout_builder\SectionListInterface
+   *   The section list.
+   */
+  abstract protected function getSectionList();
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getStorageType() {
+    return $this->getPluginId();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  #[\ReturnTypeWillChange]
+  public function count() {
+    return $this->getSectionList()->count();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getSections() {
+    return $this->getSectionList()->getSections();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getSection($delta) {
+    return $this->getSectionList()->getSection($delta);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function appendSection(Section $section) {
+    $this->getSectionList()->appendSection($section);
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function insertSection($delta, Section $section) {
+    $this->getSectionList()->insertSection($delta, $section);
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function removeSection($delta) {
+    $this->getSectionList()->removeSection($delta);
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function removeAllSections($set_blank = FALSE) {
+    $this->getSectionList()->removeAllSections($set_blank);
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getContextsDuringPreview() {
+    $contexts = $this->getContexts();
+
+    // view_mode is a required context, but SectionStorage plugins are not
+    // required to return it (for example, the layout_library plugin provided
+    // in the Layout Library module. In these instances, explicitly create a
+    // view_mode context with the value "default".
+    if (!isset($contexts['view_mode']) || $contexts['view_mode']->validate()->count() || !$contexts['view_mode']->getContextValue()) {
+      $contexts['view_mode'] = new Context(new ContextDefinition('string'), 'default');
+    }
+    return $contexts;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getTempstoreKey() {
+    return $this->getStorageId();
+  }
+
+}

core/modules/layout_builder/src/Plugin/SectionStorage/SectionStorageLocalTaskProviderInterface.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace Drupal\layout_builder\Plugin\SectionStorage;
+
+/**
+ * Allows section storage plugins to provide local tasks.
+ *
+ * @see \Drupal\layout_builder\Plugin\Derivative\LayoutBuilderLocalTaskDeriver
+ * @see \Drupal\layout_builder\SectionStorageInterface
+ */
+interface SectionStorageLocalTaskProviderInterface {
+
+  /**
+   * Provides the local tasks dynamically for Layout Builder plugins.
+   *
+   * @param mixed $base_plugin_definition
+   *   The definition of the base plugin.
+   *
+   * @return array
+   *   An array of full derivative definitions keyed on derivative ID.
+   */
+  public function buildLocalTasks($base_plugin_definition);
+
+}

core/modules/layout_builder/src/Routing/LayoutBuilderRoutes.php

@@ -0,0 +1,57 @@
+<?php
+
+namespace Drupal\layout_builder\Routing;
+
+use Drupal\Core\Routing\RouteBuildEvent;
+use Drupal\Core\Routing\RoutingEvents;
+use Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+/**
+ * Provides routes for the Layout Builder UI.
+ *
+ * @internal
+ *   Tagged services are internal.
+ */
+class LayoutBuilderRoutes implements EventSubscriberInterface {
+
+  /**
+   * The section storage manager.
+   *
+   * @var \Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface
+   */
+  protected $sectionStorageManager;
+
+  /**
+   * Constructs a new LayoutBuilderRoutes.
+   *
+   * @param \Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface $section_storage_manager
+   *   The section storage manager.
+   */
+  public function __construct(SectionStorageManagerInterface $section_storage_manager) {
+    $this->sectionStorageManager = $section_storage_manager;
+  }
+
+  /**
+   * Alters existing routes for a specific collection.
+   *
+   * @param \Drupal\Core\Routing\RouteBuildEvent $event
+   *   The route build event.
+   */
+  public function onAlterRoutes(RouteBuildEvent $event) {
+    $collection = $event->getRouteCollection();
+    foreach ($this->sectionStorageManager->getDefinitions() as $plugin_id => $definition) {
+      $this->sectionStorageManager->loadEmpty($plugin_id)->buildRoutes($collection);
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents(): array {
+    // Run after \Drupal\field_ui\Routing\RouteSubscriber.
+    $events[RoutingEvents::ALTER] = ['onAlterRoutes', -110];
+    return $events;
+  }
+
+}

core/modules/layout_builder/src/Routing/LayoutBuilderRoutesTrait.php

@@ -0,0 +1,103 @@
+<?php
+
+namespace Drupal\layout_builder\Routing;
+
+use Drupal\Component\Utility\NestedArray;
+use Drupal\layout_builder\DefaultsSectionStorageInterface;
+use Drupal\layout_builder\OverridesSectionStorageInterface;
+use Drupal\layout_builder\SectionStorage\SectionStorageDefinition;
+use Symfony\Component\Routing\Route;
+use Symfony\Component\Routing\RouteCollection;
+
+/**
+ * Provides a trait for building routes for a Layout Builder UI.
+ */
+trait LayoutBuilderRoutesTrait {
+
+  /**
+   * Builds the layout routes for the given values.
+   *
+   * @param \Symfony\Component\Routing\RouteCollection $collection
+   *   The route collection.
+   * @param \Drupal\layout_builder\SectionStorage\SectionStorageDefinition $definition
+   *   The definition of the section storage.
+   * @param string $path
+   *   The path patten for the routes.
+   * @param array $defaults
+   *   (optional) An array of default parameter values.
+   * @param array $requirements
+   *   (optional) An array of requirements for parameters.
+   * @param array $options
+   *   (optional) An array of options.
+   * @param string $route_name_prefix
+   *   (optional) The prefix to use for the route name.
+   * @param string $entity_type_id
+   *   (optional) The entity type ID, if available.
+   */
+  protected function buildLayoutRoutes(RouteCollection $collection, SectionStorageDefinition $definition, $path, array $defaults = [], array $requirements = [], array $options = [], $route_name_prefix = '', $entity_type_id = '') {
+    $type = $definition->id();
+    $defaults['section_storage_type'] = $type;
+    // Provide an empty value to allow the section storage to be upcast.
+    $defaults['section_storage'] = '';
+    // Trigger the layout builder access check.
+    $requirements['_layout_builder_access'] = 'view';
+    // Trigger the layout builder RouteEnhancer.
+    $options['_layout_builder'] = TRUE;
+    // Trigger the layout builder param converter.
+    $parameters['section_storage']['layout_builder_tempstore'] = TRUE;
+    // Merge the passed in options in after Layout Builder's parameters.
+    $options = NestedArray::mergeDeep(['parameters' => $parameters], $options);
+
+    if ($route_name_prefix) {
+      $route_name_prefix = "layout_builder.$type.$route_name_prefix";
+    }
+    else {
+      $route_name_prefix = "layout_builder.$type";
+    }
+
+    $main_defaults = $defaults;
+    $main_options = $options;
+    if ($entity_type_id) {
+      $main_defaults['_entity_form'] = "$entity_type_id.layout_builder";
+    }
+    else {
+      $main_defaults['_controller'] = '\Drupal\layout_builder\Controller\LayoutBuilderController::layout';
+    }
+    $main_defaults['_title_callback'] = '\Drupal\layout_builder\Controller\LayoutBuilderController::title';
+    $route = (new Route($path))
+      ->setDefaults($main_defaults)
+      ->setRequirements($requirements)
+      ->setOptions($main_options);
+    $collection->add("$route_name_prefix.view", $route);
+
+    $discard_changes_defaults = $defaults;
+    $discard_changes_defaults['_form'] = '\Drupal\layout_builder\Form\DiscardLayoutChangesForm';
+    $route = (new Route("$path/discard-changes"))
+      ->setDefaults($discard_changes_defaults)
+      ->setRequirements($requirements)
+      ->setOptions($options);
+    $collection->add("$route_name_prefix.discard_changes", $route);
+
+    if (is_subclass_of($definition->getClass(), OverridesSectionStorageInterface::class)) {
+      $revert_defaults = $defaults;
+      $revert_defaults['_form'] = '\Drupal\layout_builder\Form\RevertOverridesForm';
+      $route = (new Route("$path/revert"))
+        ->setDefaults($revert_defaults)
+        ->setRequirements($requirements)
+        ->setOptions($options);
+      $collection->add("$route_name_prefix.revert", $route);
+    }
+    elseif (is_subclass_of($definition->getClass(), DefaultsSectionStorageInterface::class)) {
+      $disable_defaults = $defaults;
+      $disable_defaults['_form'] = '\Drupal\layout_builder\Form\LayoutBuilderDisableForm';
+      $disable_options = $options;
+      unset($disable_options['_admin_route'], $disable_options['_layout_builder']);
+      $route = (new Route("$path/disable"))
+        ->setDefaults($disable_defaults)
+        ->setRequirements($requirements)
+        ->setOptions($disable_options);
+      $collection->add("$route_name_prefix.disable", $route);
+    }
+  }
+
+}

core/modules/layout_builder/src/Routing/LayoutSectionStorageParamConverter.php

@@ -0,0 +1,57 @@
+<?php
+
+namespace Drupal\layout_builder\Routing;
+
+use Drupal\Core\ParamConverter\ParamConverterInterface;
+use Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface;
+use Symfony\Component\Routing\Route;
+
+/**
+ * Loads the section storage from the routing defaults.
+ *
+ * @internal
+ *   Tagged services are internal.
+ */
+class LayoutSectionStorageParamConverter implements ParamConverterInterface {
+
+  /**
+   * The section storage manager.
+   *
+   * @var \Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface
+   */
+  protected $sectionStorageManager;
+
+  /**
+   * Constructs a new LayoutSectionStorageParamConverter.
+   *
+   * @param \Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface $section_storage_manager
+   *   The section storage manager.
+   */
+  public function __construct(SectionStorageManagerInterface $section_storage_manager) {
+    $this->sectionStorageManager = $section_storage_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function convert($value, $definition, $name, array $defaults) {
+    // If no section storage type is specified or if it is invalid, return.
+    if (!isset($defaults['section_storage_type']) || !$this->sectionStorageManager->hasDefinition($defaults['section_storage_type'])) {
+      return NULL;
+    }
+
+    $type = $defaults['section_storage_type'];
+    // Load an empty instance and derive the available contexts.
+    $contexts = $this->sectionStorageManager->loadEmpty($type)->deriveContextsFromRoute($value, $definition, $name, $defaults);
+    // Attempt to load a full instance based on the context.
+    return $this->sectionStorageManager->load($type, $contexts);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function applies($definition, $name, Route $route) {
+    return !empty($definition['layout_builder_section_storage']) || !empty($definition['layout_builder_tempstore']);
+  }
+
+}

core/modules/layout_builder/src/Routing/LayoutTempstoreRouteEnhancer.php

@@ -0,0 +1,44 @@
+<?php
+
+namespace Drupal\layout_builder\Routing;
+
+use Drupal\Core\Routing\EnhancerInterface;
+use Drupal\Core\Routing\RouteObjectInterface;
+use Drupal\layout_builder\LayoutTempstoreRepositoryInterface;
+use Drupal\layout_builder\SectionStorageInterface;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Loads the section storage from the layout tempstore.
+ */
+class LayoutTempstoreRouteEnhancer implements EnhancerInterface {
+
+  /**
+   * The layout tempstore repository.
+   *
+   * @var \Drupal\layout_builder\LayoutTempstoreRepositoryInterface
+   */
+  protected $layoutTempstoreRepository;
+
+  /**
+   * Constructs a new LayoutTempstoreRouteEnhancer.
+   *
+   * @param \Drupal\layout_builder\LayoutTempstoreRepositoryInterface $layout_tempstore_repository
+   *   The layout tempstore repository.
+   */
+  public function __construct(LayoutTempstoreRepositoryInterface $layout_tempstore_repository) {
+    $this->layoutTempstoreRepository = $layout_tempstore_repository;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function enhance(array $defaults, Request $request) {
+    $parameters = $defaults[RouteObjectInterface::ROUTE_OBJECT]->getOption('parameters');
+    if (isset($parameters['section_storage']['layout_builder_tempstore']) && isset($defaults['section_storage']) && $defaults['section_storage'] instanceof SectionStorageInterface) {
+      $defaults['section_storage'] = $this->layoutTempstoreRepository->get($defaults['section_storage']);
+    }
+    return $defaults;
+  }
+
+}

core/modules/layout_builder/src/Section.php

@@ -0,0 +1,454 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+use Drupal\Core\Config\Entity\ThirdPartySettingsInterface;
+use Drupal\Core\Plugin\PreviewAwarePluginInterface;
+use Drupal\Core\Render\Element;
+
+/**
+ * Provides a domain object for layout sections.
+ *
+ * A section consists of three parts:
+ * - The layout plugin ID for the layout applied to the section (for example,
+ *   'layout_onecol').
+ * - An array of settings for the layout plugin.
+ * - An array of components that can be rendered in the section.
+ *
+ * @see \Drupal\Core\Layout\LayoutDefinition
+ * @see \Drupal\layout_builder\SectionComponent
+ */
+class Section implements ThirdPartySettingsInterface {
+
+  /**
+   * The layout plugin ID.
+   *
+   * @var string
+   */
+  protected $layoutId;
+
+  /**
+   * The layout plugin settings.
+   *
+   * @var array
+   */
+  protected $layoutSettings = [];
+
+  /**
+   * An array of components, keyed by UUID.
+   *
+   * @var \Drupal\layout_builder\SectionComponent[]
+   */
+  protected $components = [];
+
+  /**
+   * Third party settings.
+   *
+   * An array of key/value pairs keyed by provider.
+   *
+   * @var array[]
+   */
+  protected $thirdPartySettings = [];
+
+  /**
+   * Constructs a new Section.
+   *
+   * @param string $layout_id
+   *   The layout plugin ID.
+   * @param array $layout_settings
+   *   (optional) The layout plugin settings.
+   * @param \Drupal\layout_builder\SectionComponent[] $components
+   *   (optional) The components.
+   * @param array[] $third_party_settings
+   *   (optional) Any third party settings.
+   */
+  public function __construct($layout_id, array $layout_settings = [], array $components = [], array $third_party_settings = []) {
+    $this->layoutId = $layout_id;
+    $this->layoutSettings = $layout_settings;
+    foreach ($components as $component) {
+      $this->setComponent($component);
+    }
+    $this->thirdPartySettings = $third_party_settings;
+  }
+
+  /**
+   * Returns the renderable array for this section.
+   *
+   * @param \Drupal\Core\Plugin\Context\ContextInterface[] $contexts
+   *   An array of available contexts.
+   * @param bool $in_preview
+   *   TRUE if the section is being previewed, FALSE otherwise.
+   *
+   * @return array
+   *   A renderable array representing the content of the section.
+   */
+  public function toRenderArray(array $contexts = [], $in_preview = FALSE) {
+    $regions = [];
+    foreach ($this->getComponents() as $component) {
+      if ($output = $component->toRenderArray($contexts, $in_preview)) {
+        $regions[$component->getRegion()][$component->getUuid()] = $output;
+      }
+    }
+
+    $layout = $this->getLayout($contexts);
+    if ($layout instanceof PreviewAwarePluginInterface) {
+      $layout->setInPreview($in_preview);
+    }
+
+    $build = $layout->build($regions);
+    // If an entity was used to build the layout, store it on the build.
+    if (!Element::isEmpty($build) && isset($contexts['layout_builder.entity'])) {
+      $build['#entity'] = $contexts['layout_builder.entity']->getContextValue();
+    }
+    return $build;
+  }
+
+  /**
+   * Gets the layout plugin for this section.
+   *
+   * @param \Drupal\Core\Plugin\Context\ContextInterface[] $contexts
+   *   An array of available contexts.
+   *
+   * @return \Drupal\Core\Layout\LayoutInterface
+   *   The layout plugin.
+   */
+  public function getLayout(array $contexts = []) {
+    $layout = $this->layoutPluginManager()->createInstance($this->getLayoutId(), $this->layoutSettings);
+    if ($contexts) {
+      $this->contextHandler()->applyContextMapping($layout, $contexts);
+    }
+    return $layout;
+  }
+
+  /**
+   * Gets the layout plugin ID for this section.
+   *
+   * @return string
+   *   The layout plugin ID.
+   *
+   * @internal
+   *   This method should only be used by code responsible for storing the data.
+   */
+  public function getLayoutId() {
+    return $this->layoutId;
+  }
+
+  /**
+   * Gets the layout plugin settings for this section.
+   *
+   * @return mixed[]
+   *   The layout plugin settings.
+   *
+   * @internal
+   *   This method should only be used by code responsible for storing the data.
+   */
+  public function getLayoutSettings() {
+    return $this->getLayout()->getConfiguration();
+  }
+
+  /**
+   * Sets the layout plugin settings for this section.
+   *
+   * @param mixed[] $layout_settings
+   *   The layout plugin settings.
+   *
+   * @return $this
+   */
+  public function setLayoutSettings(array $layout_settings) {
+    $this->layoutSettings = $layout_settings;
+    return $this;
+  }
+
+  /**
+   * Gets the default region.
+   *
+   * @return string
+   *   The machine-readable name of the default region.
+   */
+  public function getDefaultRegion() {
+    return $this->layoutPluginManager()->getDefinition($this->getLayoutId())->getDefaultRegion();
+  }
+
+  /**
+   * Returns the components of the section.
+   *
+   * @return \Drupal\layout_builder\SectionComponent[]
+   *   An array of components, keyed by the component UUID.
+   */
+  public function getComponents() {
+    return $this->components;
+  }
+
+  /**
+   * Gets the component for a given UUID.
+   *
+   * @param string $uuid
+   *   The UUID of the component to retrieve.
+   *
+   * @return \Drupal\layout_builder\SectionComponent
+   *   The component.
+   *
+   * @throws \InvalidArgumentException
+   *   Thrown when the expected UUID does not exist.
+   */
+  public function getComponent($uuid) {
+    if (!isset($this->components[$uuid])) {
+      throw new \InvalidArgumentException(sprintf('Invalid UUID "%s"', $uuid));
+    }
+
+    return $this->components[$uuid];
+  }
+
+  /**
+   * Helper method to set a component.
+   *
+   * @param \Drupal\layout_builder\SectionComponent $component
+   *   The component.
+   *
+   * @return $this
+   */
+  protected function setComponent(SectionComponent $component) {
+    $this->components[$component->getUuid()] = $component;
+    return $this;
+  }
+
+  /**
+   * Removes a given component from a region.
+   *
+   * @param string $uuid
+   *   The UUID of the component to remove.
+   *
+   * @return $this
+   */
+  public function removeComponent($uuid) {
+    unset($this->components[$uuid]);
+    return $this;
+  }
+
+  /**
+   * Appends a component to the end of a region.
+   *
+   * @param \Drupal\layout_builder\SectionComponent $component
+   *   The component being appended.
+   *
+   * @return $this
+   */
+  public function appendComponent(SectionComponent $component) {
+    $component->setWeight($this->getNextHighestWeight($component->getRegion()));
+    $this->setComponent($component);
+    return $this;
+  }
+
+  /**
+   * Returns the next highest weight of the component in a region.
+   *
+   * @param string $region
+   *   The region name.
+   *
+   * @return int
+   *   A number higher than the highest weight of the component in the region.
+   */
+  protected function getNextHighestWeight($region) {
+    $components = $this->getComponentsByRegion($region);
+    $weights = array_map(function (SectionComponent $component) {
+      return $component->getWeight();
+    }, $components);
+    return $weights ? max($weights) + 1 : 0;
+  }
+
+  /**
+   * Gets the components for a specific region.
+   *
+   * @param string $region
+   *   The region name.
+   *
+   * @return \Drupal\layout_builder\SectionComponent[]
+   *   An array of components in the specified region, sorted by weight.
+   */
+  public function getComponentsByRegion($region) {
+    $components = array_filter($this->getComponents(), function (SectionComponent $component) use ($region) {
+      return $component->getRegion() === $region;
+    });
+    uasort($components, function (SectionComponent $a, SectionComponent $b) {
+      return $a->getWeight() <=> $b->getWeight();
+    });
+    return $components;
+  }
+
+  /**
+   * Inserts a component after a specified existing component.
+   *
+   * @param string $preceding_uuid
+   *   The UUID of the existing component to insert after.
+   * @param \Drupal\layout_builder\SectionComponent $component
+   *   The component being inserted.
+   *
+   * @return $this
+   *
+   * @throws \InvalidArgumentException
+   *   Thrown when the expected UUID does not exist.
+   */
+  public function insertAfterComponent($preceding_uuid, SectionComponent $component) {
+    // Find the delta of the specified UUID.
+    $uuids = array_keys($this->getComponentsByRegion($component->getRegion()));
+    $delta = array_search($preceding_uuid, $uuids, TRUE);
+    if ($delta === FALSE) {
+      throw new \InvalidArgumentException(sprintf('Invalid preceding UUID "%s"', $preceding_uuid));
+    }
+    return $this->insertComponent($delta + 1, $component);
+  }
+
+  /**
+   * Inserts a component at a specified delta.
+   *
+   * @param int $delta
+   *   The zero-based delta in which to insert the component.
+   * @param \Drupal\layout_builder\SectionComponent $new_component
+   *   The component being inserted.
+   *
+   * @return $this
+   *
+   * @throws \OutOfBoundsException
+   *   Thrown when the specified delta is invalid.
+   */
+  public function insertComponent($delta, SectionComponent $new_component) {
+    $components = $this->getComponentsByRegion($new_component->getRegion());
+    $count = count($components);
+    if ($delta > $count) {
+      throw new \OutOfBoundsException(sprintf('Invalid delta "%s" for the "%s" component', $delta, $new_component->getUuid()));
+    }
+
+    // If the delta is the end of the list, append the component instead.
+    if ($delta === $count) {
+      return $this->appendComponent($new_component);
+    }
+
+    // Find the weight of the component that exists at the specified delta.
+    $weight = array_values($components)[$delta]->getWeight();
+    $this->setComponent($new_component->setWeight($weight++));
+
+    // Increase the weight of every subsequent component.
+    foreach (array_slice($components, $delta) as $component) {
+      $component->setWeight($weight++);
+    }
+    return $this;
+  }
+
+  /**
+   * Wraps the layout plugin manager.
+   *
+   * @return \Drupal\Core\Layout\LayoutPluginManagerInterface
+   *   The layout plugin manager.
+   */
+  protected function layoutPluginManager() {
+    return \Drupal::service('plugin.manager.core.layout');
+  }
+
+  /**
+   * Returns an array representation of the section.
+   *
+   * Only use this method if you are implementing custom storage for sections.
+   *
+   * @return array
+   *   An array representation of the section component.
+   */
+  public function toArray() {
+    return [
+      'layout_id' => $this->getLayoutId(),
+      'layout_settings' => $this->getLayoutSettings(),
+      'components' => array_map(function (SectionComponent $component) {
+        return $component->toArray();
+      }, $this->getComponents()),
+      'third_party_settings' => $this->thirdPartySettings,
+    ];
+  }
+
+  /**
+   * Creates an object from an array representation of the section.
+   *
+   * Only use this method if you are implementing custom storage for sections.
+   *
+   * @param array $section
+   *   An array of section data in the format returned by ::toArray().
+   *
+   * @return static
+   *   The section object.
+   */
+  public static function fromArray(array $section) {
+    // Ensure expected array keys are present.
+    $section += [
+      'layout_id' => '',
+      'layout_settings' => [],
+      'components' => [],
+      'third_party_settings' => [],
+    ];
+    return new static(
+      $section['layout_id'],
+      $section['layout_settings'],
+      array_map([SectionComponent::class, 'fromArray'], $section['components']),
+      $section['third_party_settings']
+    );
+  }
+
+  /**
+   * Magic method: Implements a deep clone.
+   */
+  public function __clone() {
+    foreach ($this->components as $uuid => $component) {
+      $this->components[$uuid] = clone $component;
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getThirdPartySetting($provider, $key, $default = NULL) {
+    return $this->thirdPartySettings[$provider][$key] ?? $default;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getThirdPartySettings($provider) {
+    return $this->thirdPartySettings[$provider] ?? [];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setThirdPartySetting($provider, $key, $value) {
+    $this->thirdPartySettings[$provider][$key] = $value;
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function unsetThirdPartySetting($provider, $key) {
+    unset($this->thirdPartySettings[$provider][$key]);
+    // If the third party is no longer storing any information, completely
+    // remove the array holding the settings for this provider.
+    if (empty($this->thirdPartySettings[$provider])) {
+      unset($this->thirdPartySettings[$provider]);
+    }
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getThirdPartyProviders() {
+    return array_keys($this->thirdPartySettings);
+  }
+
+  /**
+   * Wraps the context handler.
+   *
+   * @return \Drupal\Core\Plugin\Context\ContextHandlerInterface
+   *   The context handler.
+   */
+  protected function contextHandler() {
+    return \Drupal::service('context.handler');
+  }
+
+}

core/modules/layout_builder/src/SectionComponent.php

@@ -0,0 +1,320 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+use Drupal\Component\Plugin\Exception\PluginException;
+use Drupal\Core\Plugin\ContextAwarePluginInterface;
+use Drupal\layout_builder\Event\SectionComponentBuildRenderArrayEvent;
+
+/**
+ * Provides a value object for a section component.
+ *
+ * A component represents the smallest part of a layout (for example, a block).
+ * Components wrap a renderable plugin, currently using
+ * \Drupal\Core\Block\BlockPluginInterface, and contain the layout region
+ * within the section layout where the component will be rendered.
+ *
+ * @see \Drupal\Core\Layout\LayoutDefinition
+ * @see \Drupal\layout_builder\Section
+ * @see \Drupal\layout_builder\SectionStorageInterface
+ */
+class SectionComponent {
+
+  /**
+   * The UUID of the component.
+   *
+   * @var string
+   */
+  protected $uuid;
+
+  /**
+   * The region the component is placed in.
+   *
+   * @var string
+   */
+  protected $region;
+
+  /**
+   * An array of plugin configuration.
+   *
+   * @var mixed[]
+   */
+  protected $configuration;
+
+  /**
+   * The weight of the component.
+   *
+   * @var int
+   */
+  protected $weight = 0;
+
+  /**
+   * Any additional properties and values.
+   *
+   * @var mixed[]
+   */
+  protected $additional = [];
+
+  /**
+   * Constructs a new SectionComponent.
+   *
+   * @param string $uuid
+   *   The UUID.
+   * @param string $region
+   *   The region.
+   * @param mixed[] $configuration
+   *   The plugin configuration.
+   * @param mixed[] $additional
+   *   An additional values.
+   */
+  public function __construct($uuid, $region, array $configuration = [], array $additional = []) {
+    $this->uuid = $uuid;
+    $this->region = $region;
+    $this->configuration = $configuration;
+    $this->additional = $additional;
+  }
+
+  /**
+   * Returns the renderable array for this component.
+   *
+   * @param \Drupal\Core\Plugin\Context\ContextInterface[] $contexts
+   *   An array of available contexts.
+   * @param bool $in_preview
+   *   TRUE if the component is being previewed, FALSE otherwise.
+   *
+   * @return array
+   *   A renderable array representing the content of the component.
+   */
+  public function toRenderArray(array $contexts = [], $in_preview = FALSE) {
+    $event = new SectionComponentBuildRenderArrayEvent($this, $contexts, $in_preview);
+    $this->eventDispatcher()->dispatch($event, LayoutBuilderEvents::SECTION_COMPONENT_BUILD_RENDER_ARRAY);
+    $output = $event->getBuild();
+    $event->getCacheableMetadata()->applyTo($output);
+    return $output;
+  }
+
+  /**
+   * Gets any arbitrary property for the component.
+   *
+   * @param string $property
+   *   The property to retrieve.
+   *
+   * @return mixed
+   *   The value for that property, or NULL if the property does not exist.
+   */
+  public function get($property) {
+    if (property_exists($this, $property)) {
+      $value = $this->{$property} ?? NULL;
+    }
+    else {
+      $value = $this->additional[$property] ?? NULL;
+    }
+    return $value;
+  }
+
+  /**
+   * Sets a value to an arbitrary property for the component.
+   *
+   * @param string $property
+   *   The property to use for the value.
+   * @param mixed $value
+   *   The value to set.
+   *
+   * @return $this
+   */
+  public function set($property, $value) {
+    if (property_exists($this, $property)) {
+      $this->{$property} = $value;
+    }
+    else {
+      $this->additional[$property] = $value;
+    }
+    return $this;
+  }
+
+  /**
+   * Gets the region for the component.
+   *
+   * @return string
+   *   The region.
+   */
+  public function getRegion() {
+    return $this->region;
+  }
+
+  /**
+   * Sets the region for the component.
+   *
+   * @param string $region
+   *   The region.
+   *
+   * @return $this
+   */
+  public function setRegion($region) {
+    $this->region = $region;
+    return $this;
+  }
+
+  /**
+   * Gets the weight of the component.
+   *
+   * @return int
+   *   The zero-based weight of the component.
+   *
+   * @throws \UnexpectedValueException
+   *   Thrown if the weight was never set.
+   */
+  public function getWeight() {
+    return $this->weight;
+  }
+
+  /**
+   * Sets the weight of the component.
+   *
+   * @param int $weight
+   *   The zero-based weight of the component.
+   *
+   * @return $this
+   */
+  public function setWeight($weight) {
+    $this->weight = $weight;
+    return $this;
+  }
+
+  /**
+   * Gets the component plugin configuration.
+   *
+   * @return mixed[]
+   *   The component plugin configuration.
+   */
+  protected function getConfiguration() {
+    return $this->configuration;
+  }
+
+  /**
+   * Sets the plugin configuration.
+   *
+   * @param mixed[] $configuration
+   *   The plugin configuration.
+   *
+   * @return $this
+   */
+  public function setConfiguration(array $configuration) {
+    $this->configuration = $configuration;
+    return $this;
+  }
+
+  /**
+   * Gets the plugin ID.
+   *
+   * @return string
+   *   The plugin ID.
+   *
+   * @throws \Drupal\Component\Plugin\Exception\PluginException
+   *   Thrown if the plugin ID cannot be found.
+   */
+  public function getPluginId() {
+    if (empty($this->configuration['id'])) {
+      throw new PluginException(sprintf('No plugin ID specified for component with "%s" UUID', $this->uuid));
+    }
+    return $this->configuration['id'];
+  }
+
+  /**
+   * Gets the UUID for this component.
+   *
+   * @return string
+   *   The UUID.
+   */
+  public function getUuid() {
+    return $this->uuid;
+  }
+
+  /**
+   * Gets the plugin for this component.
+   *
+   * @param \Drupal\Core\Plugin\Context\ContextInterface[] $contexts
+   *   An array of contexts to set on the plugin.
+   *
+   * @return \Drupal\Component\Plugin\PluginInspectionInterface
+   *   The plugin.
+   */
+  public function getPlugin(array $contexts = []) {
+    $plugin = $this->pluginManager()->createInstance($this->getPluginId(), $this->getConfiguration());
+    if ($contexts && $plugin instanceof ContextAwarePluginInterface) {
+      $this->contextHandler()->applyContextMapping($plugin, $contexts);
+    }
+    return $plugin;
+  }
+
+  /**
+   * Wraps the component plugin manager.
+   *
+   * @return \Drupal\Core\Block\BlockManagerInterface
+   *   The plugin manager.
+   */
+  protected function pluginManager() {
+    // @todo Figure out the best way to unify fields and blocks and components
+    //   in https://www.drupal.org/node/1875974.
+    return \Drupal::service('plugin.manager.block');
+  }
+
+  /**
+   * Wraps the context handler.
+   *
+   * @return \Drupal\Core\Plugin\Context\ContextHandlerInterface
+   *   The context handler.
+   */
+  protected function contextHandler() {
+    return \Drupal::service('context.handler');
+  }
+
+  /**
+   * Wraps the event dispatcher.
+   *
+   * @return \Symfony\Contracts\EventDispatcher\EventDispatcherInterface
+   *   The event dispatcher.
+   */
+  protected function eventDispatcher() {
+    return \Drupal::service('event_dispatcher');
+  }
+
+  /**
+   * Returns an array representation of the section component.
+   *
+   * Only use this method if you are implementing custom storage for sections.
+   *
+   * @return array
+   *   An array representation of the section component.
+   */
+  public function toArray() {
+    return [
+      'uuid' => $this->getUuid(),
+      'region' => $this->getRegion(),
+      'configuration' => $this->getConfiguration(),
+      'weight' => $this->getWeight(),
+      'additional' => $this->additional,
+    ];
+  }
+
+  /**
+   * Creates an object from an array representation of the section component.
+   *
+   * Only use this method if you are implementing custom storage for sections.
+   *
+   * @param array $component
+   *   An array of section component data in the format returned by ::toArray().
+   *
+   * @return static
+   *   The section component object.
+   */
+  public static function fromArray(array $component) {
+    return (new static(
+      $component['uuid'],
+      $component['region'],
+      $component['configuration'],
+      $component['additional']
+    ))->setWeight($component['weight']);
+  }
+
+}

core/modules/layout_builder/src/SectionListInterface.php

@@ -0,0 +1,84 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+/**
+ * Defines the interface for an object that stores layout sections.
+ *
+ * @see \Drupal\layout_builder\Section
+ */
+interface SectionListInterface extends \Countable {
+
+  /**
+   * Gets the layout sections.
+   *
+   * @return \Drupal\layout_builder\Section[]
+   *   A sequentially and numerically keyed array of section objects.
+   */
+  public function getSections();
+
+  /**
+   * Gets a domain object for the layout section.
+   *
+   * @param int $delta
+   *   The delta of the section.
+   *
+   * @return \Drupal\layout_builder\Section
+   *   The layout section.
+   */
+  public function getSection($delta);
+
+  /**
+   * Appends a new section to the end of the list.
+   *
+   * @param \Drupal\layout_builder\Section $section
+   *   The section to append.
+   *
+   * @return $this
+   */
+  public function appendSection(Section $section);
+
+  /**
+   * Inserts a new section at a given delta.
+   *
+   * If a section exists at the given index, the section at that position and
+   * others after it are shifted backward.
+   *
+   * @param int $delta
+   *   The delta of the section.
+   * @param \Drupal\layout_builder\Section $section
+   *   The section to insert.
+   *
+   * @return $this
+   */
+  public function insertSection($delta, Section $section);
+
+  /**
+   * Removes the section at the given delta.
+   *
+   * As sections are stored sequentially and numerically this will re-key every
+   * subsequent section, shifting them forward.
+   *
+   * @param int $delta
+   *   The delta of the section.
+   *
+   * @return $this
+   */
+  public function removeSection($delta);
+
+  /**
+   * Removes all of the sections.
+   *
+   * @param bool $set_blank
+   *   (optional) The default implementation of section lists differentiates
+   *   between a list that has never contained any sections and a list that has
+   *   purposefully had all sections removed in order to remain blank. Passing
+   *   TRUE will mirror ::removeSection() by tracking this as a blank list.
+   *   Passing FALSE will reset the list as though it had never contained any
+   *   sections at all. Defaults to FALSE.
+   *
+   * @return $this
+   */
+  public function removeAllSections($set_blank = FALSE);
+
+}

core/modules/layout_builder/src/SectionListTrait.php

@@ -0,0 +1,186 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+/**
+ * Provides a trait for maintaining a list of sections.
+ *
+ * @see \Drupal\layout_builder\SectionListInterface
+ */
+trait SectionListTrait {
+
+  /**
+   * Stores the information for all sections.
+   *
+   * Implementations of this method are expected to call array_values() to rekey
+   * the list of sections.
+   *
+   * @param \Drupal\layout_builder\Section[] $sections
+   *   An array of section objects.
+   *
+   * @return $this
+   */
+  abstract protected function setSections(array $sections);
+
+  /**
+   * {@inheritdoc}
+   */
+  #[\ReturnTypeWillChange]
+  public function count() {
+    if ($this->hasBlankSection()) {
+      return 0;
+    }
+
+    return count($this->getSections());
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getSection($delta) {
+    if (!$this->hasSection($delta)) {
+      throw new \OutOfBoundsException(sprintf('Invalid delta "%s"', $delta));
+    }
+
+    return $this->getSections()[$delta];
+  }
+
+  /**
+   * Sets the section for the given delta on the display.
+   *
+   * @param int $delta
+   *   The delta of the section.
+   * @param \Drupal\layout_builder\Section $section
+   *   The layout section.
+   *
+   * @return $this
+   */
+  protected function setSection($delta, Section $section) {
+    $sections = $this->getSections();
+    $sections[$delta] = $section;
+    $this->setSections($sections);
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function appendSection(Section $section) {
+    $delta = $this->count();
+
+    $this->setSection($delta, $section);
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function insertSection($delta, Section $section) {
+    // Clear the section list if there is currently a blank section.
+    if ($this->hasBlankSection()) {
+      $this->removeAllSections();
+    }
+
+    if ($this->hasSection($delta)) {
+      // @todo Use https://www.drupal.org/node/66183 once resolved.
+      $start = array_slice($this->getSections(), 0, $delta);
+      $end = array_slice($this->getSections(), $delta);
+      $this->setSections(array_merge($start, [$section], $end));
+    }
+    else {
+      $this->appendSection($section);
+    }
+    return $this;
+  }
+
+  /**
+   * Adds a blank section to the list.
+   *
+   * @return $this
+   *
+   * @see \Drupal\layout_builder\Plugin\Layout\BlankLayout
+   */
+  protected function addBlankSection() {
+    if ($this->hasSection(0)) {
+      throw new \Exception('A blank section must only be added to an empty list');
+    }
+
+    $this->appendSection(new Section('layout_builder_blank'));
+    return $this;
+  }
+
+  /**
+   * Indicates if this section list contains a blank section.
+   *
+   * A blank section is used to differentiate the difference between a layout
+   * that has never been instantiated and one that has purposefully had all
+   * sections removed.
+   *
+   * @return bool
+   *   TRUE if the section list contains a blank section, FALSE otherwise.
+   *
+   * @see \Drupal\layout_builder\Plugin\Layout\BlankLayout
+   */
+  protected function hasBlankSection() {
+    // A blank section will only ever exist when the delta is 0, as added by
+    // ::removeSection().
+    return $this->hasSection(0) && $this->getSection(0)->getLayoutId() === 'layout_builder_blank';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function removeSection($delta) {
+    // Clear the section list if there is currently a blank section.
+    if ($this->hasBlankSection()) {
+      $this->removeAllSections();
+    }
+
+    $sections = $this->getSections();
+    unset($sections[$delta]);
+    $this->setSections($sections);
+    // Add a blank section when the last section is removed.
+    if (empty($sections)) {
+      $this->addBlankSection();
+    }
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function removeAllSections($set_blank = FALSE) {
+    $this->setSections([]);
+    if ($set_blank) {
+      $this->addBlankSection();
+    }
+    return $this;
+  }
+
+  /**
+   * Indicates if there is a section at the specified delta.
+   *
+   * @param int $delta
+   *   The delta of the section.
+   *
+   * @return bool
+   *   TRUE if there is a section for this delta, FALSE otherwise.
+   */
+  protected function hasSection($delta) {
+    return isset($this->getSections()[$delta]);
+  }
+
+  /**
+   * Magic method: Implements a deep clone.
+   */
+  public function __clone() {
+    $sections = $this->getSections();
+
+    foreach ($sections as $delta => $item) {
+      $sections[$delta] = clone $item;
+    }
+
+    $this->setSections($sections);
+  }
+
+}

core/modules/layout_builder/src/SectionStorage/SectionStorageDefinition.php

@@ -0,0 +1,101 @@
+<?php
+
+namespace Drupal\layout_builder\SectionStorage;
+
+use Drupal\Component\Plugin\Definition\ContextAwarePluginDefinitionInterface;
+use Drupal\Component\Plugin\Definition\ContextAwarePluginDefinitionTrait;
+use Drupal\Component\Plugin\Definition\PluginDefinition;
+
+/**
+ * Provides section storage type plugin definition.
+ */
+class SectionStorageDefinition extends PluginDefinition implements ContextAwarePluginDefinitionInterface {
+
+  use ContextAwarePluginDefinitionTrait;
+
+  /**
+   * The plugin weight.
+   *
+   * @var int
+   */
+  protected $weight = 0;
+
+  /**
+   * Any additional properties and values.
+   *
+   * @var array
+   */
+  protected $additional = [];
+
+  /**
+   * LayoutDefinition constructor.
+   *
+   * @param array $definition
+   *   An array of values from the annotation.
+   */
+  public function __construct(array $definition = []) {
+    // If there are context definitions in the plugin definition, they should
+    // be added to this object using ::addContextDefinition() so that they can
+    // be manipulated using other ContextAwarePluginDefinitionInterface methods.
+    if (isset($definition['context_definitions'])) {
+      foreach ($definition['context_definitions'] as $name => $context_definition) {
+        $this->addContextDefinition($name, $context_definition);
+      }
+      unset($definition['context_definitions']);
+    }
+
+    foreach ($definition as $property => $value) {
+      $this->set($property, $value);
+    }
+  }
+
+  /**
+   * Gets any arbitrary property.
+   *
+   * @param string $property
+   *   The property to retrieve.
+   *
+   * @return mixed
+   *   The value for that property, or NULL if the property does not exist.
+   */
+  public function get($property) {
+    if (property_exists($this, $property)) {
+      $value = $this->{$property} ?? NULL;
+    }
+    else {
+      $value = $this->additional[$property] ?? NULL;
+    }
+    return $value;
+  }
+
+  /**
+   * Sets a value to an arbitrary property.
+   *
+   * @param string $property
+   *   The property to use for the value.
+   * @param mixed $value
+   *   The value to set.
+   *
+   * @return $this
+   */
+  public function set($property, $value) {
+    if (property_exists($this, $property)) {
+      $this->{$property} = $value;
+    }
+    else {
+      $this->additional[$property] = $value;
+    }
+    return $this;
+  }
+
+  /**
+   * Returns the plugin weight.
+   *
+   * @return int
+   *   The plugin weight.
+   */
+  public function getWeight() {
+    return $this->weight;
+  }
+
+}

core/modules/layout_builder/src/SectionStorage/SectionStorageManager.php

@@ -0,0 +1,110 @@
+<?php
+
+namespace Drupal\layout_builder\SectionStorage;
+
+use Drupal\Component\Plugin\Exception\ContextException;
+use Drupal\Core\Cache\CacheBackendInterface;
+use Drupal\Core\Cache\RefinableCacheableDependencyInterface;
+use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Core\Plugin\Context\ContextHandlerInterface;
+use Drupal\Core\Plugin\DefaultPluginManager;
+use Drupal\layout_builder\Attribute\SectionStorage;
+use Drupal\layout_builder\SectionStorageInterface;
+
+/**
+ * Provides the Section Storage type plugin manager.
+ *
+ * Note that while this class extends \Drupal\Core\Plugin\DefaultPluginManager
+ * and includes many additional public methods, only some of them are available
+ * via \Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface.
+ * While internally depending on the parent class is necessary, external code
+ * should only use the methods available on that interface.
+ */
+class SectionStorageManager extends DefaultPluginManager implements SectionStorageManagerInterface {
+
+  /**
+   * The context handler.
+   *
+   * @var \Drupal\Core\Plugin\Context\ContextHandlerInterface
+   */
+  protected $contextHandler;
+
+  /**
+   * Constructs a new SectionStorageManager object.
+   *
+   * @param \Traversable $namespaces
+   *   An object that implements \Traversable which contains the root paths
+   *   keyed by the corresponding namespace to look for plugin implementations.
+   * @param \Drupal\Core\Cache\CacheBackendInterface $cache_backend
+   *   Cache backend instance to use.
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
+   *   The module handler to invoke the alter hook with.
+   * @param \Drupal\Core\Plugin\Context\ContextHandlerInterface $context_handler
+   *   The context handler.
+   */
+  public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, ModuleHandlerInterface $module_handler, ContextHandlerInterface $context_handler) {
+    parent::__construct('Plugin/SectionStorage', $namespaces, $module_handler, SectionStorageInterface::class, SectionStorage::class, '\Drupal\layout_builder\Annotation\SectionStorage');
+
+    $this->contextHandler = $context_handler;
+
+    $this->alterInfo('layout_builder_section_storage');
+    $this->setCacheBackend($cache_backend, 'layout_builder_section_storage_plugins');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function findDefinitions() {
+    $definitions = parent::findDefinitions();
+
+    // Sort the definitions by their weight while preserving the original order
+    // for those with matching weights.
+    $weights = array_map(function (SectionStorageDefinition $definition) {
+      return $definition->getWeight();
+    }, $definitions);
+    $ids = array_keys($definitions);
+    array_multisort($weights, $ids, $definitions);
+    return $definitions;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function load($type, array $contexts = []) {
+    $plugin = $this->loadEmpty($type);
+    try {
+      $this->contextHandler->applyContextMapping($plugin, $contexts);
+    }
+    catch (ContextException $e) {
+      return NULL;
+    }
+    return $plugin;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function findByContext(array $contexts, RefinableCacheableDependencyInterface $cacheability) {
+    $storage_types = array_keys($this->contextHandler->filterPluginDefinitionsByContexts($contexts, $this->getDefinitions()));
+
+    // Add the manager as a cacheable dependency in order to vary by changes to
+    // the plugin definitions.
+    $cacheability->addCacheableDependency($this);
+
+    foreach ($storage_types as $type) {
+      $plugin = $this->load($type, $contexts);
+      if ($plugin && $plugin->isApplicable($cacheability)) {
+        return $plugin;
+      }
+    }
+    return NULL;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function loadEmpty($type) {
+    return $this->createInstance($type);
+  }
+
+}

core/modules/layout_builder/src/SectionStorage/SectionStorageManagerInterface.php

@@ -0,0 +1,62 @@
+<?php
+
+namespace Drupal\layout_builder\SectionStorage;
+
+use Drupal\Component\Plugin\Discovery\DiscoveryInterface;
+use Drupal\Core\Cache\RefinableCacheableDependencyInterface;
+
+/**
+ * Provides the interface for a plugin manager of section storage types.
+ */
+interface SectionStorageManagerInterface extends DiscoveryInterface {
+
+  /**
+   * Loads a section storage with the provided contexts applied.
+   *
+   * @param string $type
+   *   The section storage type.
+   * @param \Drupal\Component\Plugin\Context\ContextInterface[] $contexts
+   *   (optional) The contexts available for this storage to use.
+   *
+   * @return \Drupal\layout_builder\SectionStorageInterface|null
+   *   The section storage or NULL if its context requirements are not met.
+   */
+  public function load($type, array $contexts = []);
+
+  /**
+   * Finds the section storage to load based on available contexts.
+   *
+   * @param \Drupal\Component\Plugin\Context\ContextInterface[] $contexts
+   *   The contexts which should be used to determine which storage to return.
+   * @param \Drupal\Core\Cache\RefinableCacheableDependencyInterface $cacheability
+   *   Refinable cacheability object, which will be populated based on the
+   *   cacheability of each section storage candidate. After calling this method
+   *   this parameter will reflect the cacheability information used to
+   *   determine the correct section storage. This must be associated with any
+   *   output that uses the result of this method.
+   *
+   * @return \Drupal\layout_builder\SectionStorageInterface|null
+   *   The section storage if one matched all contexts, or NULL otherwise.
+   *
+   * @see \Drupal\Core\Cache\RefinableCacheableDependencyInterface
+   */
+  public function findByContext(array $contexts, RefinableCacheableDependencyInterface $cacheability);
+
+  /**
+   * Loads a section storage with no associated section list.
+   *
+   * @param string $type
+   *   The type of the section storage being instantiated.
+   *
+   * @return \Drupal\layout_builder\SectionStorageInterface
+   *   The section storage.
+   *
+   * @internal
+   *   Section storage relies on context to load section lists. Use ::load()
+   *   when that context is available. This method is intended for use by
+   *   collaborators of the plugins in build-time situations when section
+   *   storage type must be consulted.
+   */
+  public function loadEmpty($type);
+
+}

core/modules/layout_builder/src/SectionStorageInterface.php

@@ -0,0 +1,153 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+use Drupal\Component\Plugin\PluginInspectionInterface;
+use Drupal\Core\Access\AccessibleInterface;
+use Drupal\Core\Cache\RefinableCacheableDependencyInterface;
+use Drupal\Core\Plugin\ContextAwarePluginInterface;
+use Drupal\Core\Session\AccountInterface;
+use Symfony\Component\Routing\RouteCollection;
+
+/**
+ * Defines an interface for Section Storage type plugins.
+ */
+interface SectionStorageInterface extends SectionListInterface, PluginInspectionInterface, ContextAwarePluginInterface, AccessibleInterface {
+
+  /**
+   * Returns an identifier for this storage.
+   *
+   * @return string
+   *   The unique identifier for this storage.
+   */
+  public function getStorageId();
+
+  /**
+   * Returns the type of this storage.
+   *
+   * Used in conjunction with the storage ID.
+   *
+   * @return string
+   *   The type of storage.
+   */
+  public function getStorageType();
+
+  /**
+   * Provides the routes needed for Layout Builder UI.
+   *
+   * Allows the plugin to add or alter routes during the route building process.
+   * \Drupal\layout_builder\Routing\LayoutBuilderRoutesTrait is provided for the
+   * typical use case of building a standard Layout Builder UI.
+   *
+   * @param \Symfony\Component\Routing\RouteCollection $collection
+   *   The route collection.
+   *
+   * @see \Drupal\Core\Routing\RoutingEvents::ALTER
+   */
+  public function buildRoutes(RouteCollection $collection);
+
+  /**
+   * Gets the URL used when redirecting away from the Layout Builder UI.
+   *
+   * @return \Drupal\Core\Url
+   *   The URL object.
+   */
+  public function getRedirectUrl();
+
+  /**
+   * Gets the URL used to display the Layout Builder UI.
+   *
+   * @param string $rel
+   *   (optional) The link relationship type, for example: 'view' or 'disable'.
+   *   Defaults to 'view'.
+   *
+   * @return \Drupal\Core\Url
+   *   The URL object.
+   */
+  public function getLayoutBuilderUrl($rel = 'view');
+
+  /**
+   * Derives the available plugin contexts from route values.
+   *
+   * This should only be called during section storage instantiation,
+   * specifically for use by the routing system. For all non-routing usages, use
+   * \Drupal\Component\Plugin\ContextAwarePluginInterface::getContextValue().
+   *
+   * @param mixed $value
+   *   The raw value.
+   * @param mixed $definition
+   *   The parameter definition provided in the route options.
+   * @param string $name
+   *   The name of the parameter.
+   * @param array $defaults
+   *   The route defaults array.
+   *
+   * @return \Drupal\Core\Plugin\Context\ContextInterface[]
+   *   The available plugin contexts.
+   *
+   * @see \Drupal\Core\ParamConverter\ParamConverterInterface::convert()
+   */
+  public function deriveContextsFromRoute($value, $definition, $name, array $defaults);
+
+  /**
+   * Gets contexts for use during preview.
+   *
+   * When not in preview, ::getContexts() will be used.
+   *
+   * @return \Drupal\Core\Plugin\Context\ContextInterface[]
+   *   The plugin contexts suitable for previewing.
+   */
+  public function getContextsDuringPreview();
+
+  /**
+   * Gets the label for the object using the sections.
+   *
+   * @return string
+   *   The label, or NULL if there is no label defined.
+   */
+  public function label();
+
+  /**
+   * Saves the sections.
+   *
+   * @return int
+   *   SAVED_NEW or SAVED_UPDATED is returned depending on the operation
+   *   performed.
+   */
+  public function save();
+
+  /**
+   * Determines if this section storage is applicable for the current contexts.
+   *
+   * @param \Drupal\Core\Cache\RefinableCacheableDependencyInterface $cacheability
+   *   Refinable cacheability object, typically provided by the section storage
+   *   manager. When implementing this method, populate $cacheability with any
+   *   information that affects whether this storage is applicable.
+   *
+   * @return bool
+   *   TRUE if this section storage is applicable, FALSE otherwise.
+   *
+   * @internal
+   *   This method is intended to be called by
+   *   \Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface::findByContext().
+   *
+   * @see \Drupal\Core\Cache\RefinableCacheableDependencyInterface
+   */
+  public function isApplicable(RefinableCacheableDependencyInterface $cacheability);
+
+  /**
+   * Overrides \Drupal\Component\Plugin\PluginInspectionInterface::getPluginDefinition().
+   *
+   * @return \Drupal\layout_builder\SectionStorage\SectionStorageDefinition
+   *   The section storage definition.
+   */
+  public function getPluginDefinition();
+
+  /**
+   * Overrides \Drupal\Core\Access\AccessibleInterface::access().
+   *
+   * @ingroup layout_builder_access
+   */
+  public function access($operation, ?AccountInterface $account = NULL, $return_as_object = FALSE);
+
+}

core/modules/layout_builder/src/TempStoreIdentifierInterface.php

@@ -0,0 +1,20 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+/**
+ * Provides an interface that allows an object to provide its own tempstore key.
+ *
+ * @todo Move to \Drupal\Core\TempStore in https://www.drupal.org/node/3026957.
+ */
+interface TempStoreIdentifierInterface {
+
+  /**
+   * Gets a string suitable for use as a tempstore key.
+   *
+   * @return string
+   *   A string to be used as the key for a tempstore item.
+   */
+  public function getTempstoreKey();
+
+}