Refactor inbox_block_script library.

Closes #3491252

js/private_message_inbox_block.js

@@ -84,11 +84,10 @@
      *   HTML content of threads.
      */
     insertPreviousThreads(threadsHtml) {
-      const newNodes =
-        Drupal.PrivateMessageUtils.parseHTML(threadsHtml).childNodes;
+      const newNodes = Drupal.PrivateMessageUtils.parseHTML(threadsHtml);
 
-      newNodes.forEach((node) => {
-        const appendedElement = this.container.appendChild(node.cloneNode(true));
+      Array.from(newNodes).forEach((node) => {
+        const appendedElement = this.container.appendChild(node);
 
         Drupal.attachBehaviors(appendedElement);
         Drupal.PrivateMessageSlide.toggleSlide(appendedElement, true);
@@ -152,8 +151,8 @@
           }
           const newThreadContent = Drupal.PrivateMessageUtils.parseHTML(
             newThreads[threadId],
-          ).childNodes;
-          newThreadContent.forEach((child) => {
+          );
+          Array.from(newThreadContent).forEach((child) => {
             const appendedElement = this.container.appendChild(child);
             Drupal.attachBehaviors(appendedElement);
           });