Fix Layout translation form reports unsaved changes and becomes stale + call undefined method idDefaultTranslation()

Refs: #3222423-unsaved-changes

Closes #3222423

src/EventSubscriber/PrepareLayout.php

+<?php
+
+namespace Drupal\layout_builder_st\EventSubscriber;
+
+use Drupal\layout_builder\Event\PrepareLayoutEvent;
+use Drupal\layout_builder\EventSubscriber\PrepareLayout as CorePrepareLayout;
+
+/**
+ * Overrides the core PrepareLayout event subscriber.
+ */
+class PrepareLayout extends CorePrepareLayout {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function onPrepareLayout(PrepareLayoutEvent $event) {
+    $section_storage = $event->getSectionStorage();
+
+    // If the layout is being edited in its source language, let core
+    // layout_builder do its standard handling of the event.
+    if (method_exists($section_storage, 'isDefaultTranslation')) {
+      if ($section_storage->isDefaultTranslation()) {
+        return parent::onPrepareLayout($event);
+      }
+    }
+
+    // If a symmetric translation is being edited, do not add the section
+    // storage to the tempstore. This can result in the symmetric translation
+    // getting stuck in the "you have unsaved changes" state after the Layout
+    // Builder form is visited, even if no changes are made. Just show the
+    // unsaved changes message if needed.
+    if ($this->layoutTempstoreRepository->has($section_storage)) {
+      $this->messenger->addWarning($this->t('You have unsaved changes.'));
+    }
+  }
+
+}