Backport

Closes #3405345

core/includes/theme.inc

@@ -1320,11 +1320,30 @@ function template_preprocess_html(&$variables) {
     $variables['page']['#title'] = (string) \Drupal::service('renderer')->render($variables['page']['#title']);
   }
   if (!empty($variables['page']['#title'])) {
-    $head_title = [
-      // Marking the title as safe since it has had the tags stripped.
-      'title' => Markup::create(trim(strip_tags($variables['page']['#title']))),
-      'name' => $site_config->get('name'),
-    ];
+    $head_title = [];
+
+    // Marking the title as safe since it has had the tags stripped.
+    $head_title['title'] = Markup::create(trim(strip_tags($variables['page']['#title'])));
+
+    $maintenance_mode = defined('MAINTENANCE_MODE') || \Drupal::state()->get('system.maintenance_mode');
+    if (!$maintenance_mode) {
+      $base_route_title = \Drupal::service('base_route_title_resolver')->getTitle(\Drupal::requestStack()->getCurrentRequest(), \Drupal::routeMatch()->getRouteObject());
+      if (is_array($base_route_title)) {
+        $base_route_title = (string) \Drupal::service('renderer')->render($base_route_title);
+      }
+      if ($base_route_title) {
+        // Marking the title as safe since it has had the tags stripped.
+        $base_route_title = Markup::create(trim(strip_tags($base_route_title)));
+
+        // If the base route title is not the same as the current title, append it
+        // to the head title.
+        if ((string) $base_route_title !== (string) $head_title['title']) {
+          $head_title['base_route_title'] = $base_route_title;
+        }
+      }
+    }
+
+    $head_title['name'] = $site_config->get('name');
   }
   // @todo Remove once views is not bypassing the view subscriber anymore.
   //   @see https://www.drupal.org/node/2068471