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core/themes/claro/js/classy/README.txt

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+WHAT IS THIS DIRECTORY FOR?
+--------------------------------
+This directory is for JS files previously inherited from the Classy theme.
+
+WHY WERE CLASSY JS FILES COPIED HERE?
+-------------------------------------------
+Classy was removed in Drupal 10. To prepare for Classy's removal, JS files that
+would otherwise be inherited from Classy were copied here.

core/themes/claro/js/classy/media_embed_ckeditor.theme.js

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+/**
+ * @file
+ * Theme overrides for the Media Embed CKEditor plugin previously provided by
+ * the now-removed Classy theme.
+ */
+
+((Drupal) => {
+  /**
+   * Themes the error displayed when the media embed preview fails.
+   *
+   * @param {string} error
+   *   The error message to display
+   *
+   * @return {string}
+   *   A string representing a DOM fragment.
+   *
+   * @see media-embed-error.html.twig
+   */
+  Drupal.theme.mediaEmbedPreviewError = () =>
+    `<div class="media-embed-error media-embed-error--preview-error">${Drupal.t(
+      'An error occurred while trying to preview the media. Save your work and reload this page.',
+    )}</div>`;
+})(Drupal);