core/includes/theme.inc                                   |  9 +++++++--
 core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php  | 15 ++++++++++++++-
 .../system/src/Plugin/Block/SystemPageTitleBlock.php      |  2 +-
 3 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 0cf5fa2..1d297ab 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -1234,8 +1234,13 @@ function template_preprocess_html(&$variables) {
   $site_config = \Drupal::config('system.site');
   // Construct page title.
   if (isset($variables['page']['#title']) && is_array($variables['page']['#title'])) {
-    // Do an early render if the title is a render array.
-    $variables['page']['#title'] = (string) \Drupal::service('renderer')->render($variables['page']['#title']);
+    if (isset($variables['page']['#title']['#plain_text'])) {
+      $variables['page']['#title'] = $variables['page']['#title']['#plain_text'];
+    }
+    else {
+      // Do an early render if the title is a render array.
+      $variables['page']['#title'] = (string) \Drupal::service('renderer')->render($variables['page']['#title']);
+    }
   }
   if (!empty($variables['page']['#title'])) {
     $head_title = array(
diff --git a/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php b/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php
index cd0d482..2a80fd4 100644
--- a/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php
+++ b/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php
@@ -8,6 +8,7 @@
 namespace Drupal\Core\Render\MainContent;
 
 use Drupal\Component\Plugin\PluginManagerInterface;
+use Drupal\Component\Utility\SafeStringInterface;
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Controller\TitleResolverInterface;
 use Drupal\Core\Display\PageVariantInterface;
@@ -195,7 +196,19 @@ protected function prepare(array $main_content, Request $request, RouteMatchInte
     // Determine the title: use the title provided by the main content if any,
     // otherwise get it from the routing information.
     $get_title = function (array $main_content) use ($request, $route_match) {
-      return isset($main_content['#title']) ? $main_content['#title'] : $this->titleResolver->getTitle($request, $route_match->getRouteObject());
+      $title = isset($main_content['#title']) ? $main_content['#title'] : $this->titleResolver->getTitle($request, $route_match->getRouteObject());
+      // Ensure the title is always a render array, so we can associate the
+      // main content's cacheability with it.
+      if ($title instanceof SafeStringInterface) {
+        $title = ['#markup' => $title];
+      }
+      elseif (is_string($title)) {
+        $title = ['#plain_text' => $title];
+      }
+      if (isset($main_content['#cache'])) {
+        $title['#cache'] = $main_content['#cache'];
+      }
+      return $title;
     };
 
     // If the _controller result already is #type => page,
diff --git a/core/modules/system/src/Plugin/Block/SystemPageTitleBlock.php b/core/modules/system/src/Plugin/Block/SystemPageTitleBlock.php
index 0f431df..0e82940 100644
--- a/core/modules/system/src/Plugin/Block/SystemPageTitleBlock.php
+++ b/core/modules/system/src/Plugin/Block/SystemPageTitleBlock.php
@@ -47,7 +47,7 @@ public function defaultConfiguration() {
    * {@inheritdoc}
    */
   public function build() {
-    if (empty($this->title) || (is_array($this->title) && empty($this->title['#markup']))) {
+    if (empty($this->title) || (is_array($this->title) && empty($this->title['#markup']) && empty($this->title['#plain_text']))) {
       return [];
     }
     return [