#3412644 Whitespace HTML entities break DOM parsing in Drupal 10.2

Opening second MR as original approach was reverted in !11

src/TocBuilder.php

@@ -87,7 +87,17 @@ class TocBuilder implements TocBuilderInterface {
 
       $fragment_node = $dom->createDocumentFragment();
       $html = Html::normalize($this->renderer->render($build));
-      $fragment_node->appendXML($html);
+      // Trim to ensure the first child is a DOMElement.
+      $html = trim($html);
+      // Build a new DOMDocument for the HTML.
+      $fragment_dom = Html::load($html);
+      // Get the body element of the new document.
+      $fragment_body = $fragment_dom->getElementsByTagName('body')->item(0);
+      // Import the new (header) nodes onto the original DOMDocument.
+      foreach ($fragment_body->childNodes as $fragment_child_node) {
+        $fragment_child_node = $dom->importNode($fragment_child_node, true);
+        $fragment_node->appendChild($fragment_child_node);
+      }
 
       // Replace the header node.
       $replace = TRUE;
@@ -112,9 +122,21 @@ class TocBuilder implements TocBuilderInterface {
         '#toc' => $toc,
         '#item' => NULL,
       ];
+
       $fragment_node = $dom->createDocumentFragment();
       $html = Html::normalize($this->renderer->render($build));
-      $fragment_node->appendXML($html);
+      // Trim to ensure the first child is a DOMElement.
+      $html = trim($html);
+      // Build a new DOMDocument for the HTML.
+      $fragment_dom = Html::load($html);
+      // Get the body element of the new document.
+      $fragment_body = $fragment_dom->getElementsByTagName('body')->item(0);
+      // Import the new (header) nodes onto the original DOMDocument.
+      foreach ($fragment_body->childNodes as $fragment_child_node) {
+        $fragment_child_node = $dom->importNode($fragment_child_node, true);
+        $fragment_node->appendChild($fragment_child_node);
+      }
+
       if (!empty($fragment_node->textContent)) {
         $dom->getElementsByTagName('body')->item(0)->appendChild($fragment_node);
       }