diff -u b/core/lib/Drupal/Core/Page/DefaultHtmlPageRenderer.php b/core/lib/Drupal/Core/Page/DefaultHtmlPageRenderer.php
--- b/core/lib/Drupal/Core/Page/DefaultHtmlPageRenderer.php
+++ b/core/lib/Drupal/Core/Page/DefaultHtmlPageRenderer.php
@@ -40,8 +40,8 @@
 
     $page_content['main'] = array(
       '#markup' => $fragment->getContent(),
-      '#title' => $page->getTitle(),
     );
+    $page_content['#title'] = $page->getTitle();
 
     $page_array = drupal_prepare_page($page_content);
 
only in patch2:
unchanged:
--- a/core/modules/comment/lib/Drupal/comment/CommentFormController.php
+++ b/core/modules/comment/lib/Drupal/comment/CommentFormController.php
@@ -244,6 +244,10 @@ protected function actions(array $form, array &$form_state) {
     // already previewing the submission.
     $element['submit']['#access'] = ($comment->id() && $this->currentUser->hasPermission('administer comments')) || $preview_mode != DRUPAL_REQUIRED || isset($form_state['comment_preview']);
 
+    if (isset($form_state['triggering_element']) && $form_state['triggering_element']['#value'] == t('Preview')) {
+      $element['#title'] = $this->t('Preview comment');
+    }
+
     $element['preview'] = array(
       '#type' => 'submit',
       '#value' => $this->t('Preview'),
@@ -353,9 +357,8 @@ public function submit(array $form, array &$form_state) {
    * @param $form_state
    *   A reference to a keyed array containing the current state of the form.
    */
-  public function preview(array $form, array &$form_state) {
+  public function preview(array &$form, array &$form_state) {
     $comment = $this->entity;
-    drupal_set_title(t('Preview comment'), PASS_THROUGH);
     $form_state['comment_preview'] = comment_preview($comment);
     $form_state['rebuild'] = TRUE;
   }