diff --git a/core/lib/Drupal/Core/Extension/UpdateModuleHandler.php b/core/lib/Drupal/Core/Extension/UpdateModuleHandler.php
index 5fcf990..7b69ad4 100644
--- a/core/lib/Drupal/Core/Extension/UpdateModuleHandler.php
+++ b/core/lib/Drupal/Core/Extension/UpdateModuleHandler.php
@@ -24,6 +24,16 @@ public function getImplementations($hook) {
     if (substr($hook, -6) === '_alter') {
       return array();
     }
+    // Allow system_theme_suggestions_maintenance_page() for the offline
+    // maintenance page theme suggestion.
+    if ($hook == 'theme_suggestions_maintenance_page') {
+      return array('system');
+    }
+    // theme() is called during updates and fires suggestion hooks but we do
+    // not allow this during updates.
+    if (strpos($hook, 'theme_suggestions_') !== FALSE) {
+      return array();
+    }
     switch ($hook) {
       // hook_requirements is necessary for updates to work.
       case 'requirements':
diff --git a/core/modules/forum/forum.module b/core/modules/forum/forum.module
index d592718..16aee73 100644
--- a/core/modules/forum/forum.module
+++ b/core/modules/forum/forum.module
@@ -585,22 +585,23 @@ function forum_preprocess_block(&$variables) {
  */
 function forum_theme_suggestions_forums(array $variables) {
   $suggestions = array();
+  $tid = $variables['term']->id();
 
   // Provide separate template suggestions based on what's being output. Topic
   // ID is also accounted for. Check both variables to be safe then the inverse.
   // Forums with topic IDs take precedence.
   if ($variables['forums'] && !$variables['topics']) {
     $suggestions[] = 'forums__containers';
-    $suggestions[] = 'forums__' . $variables['tid'];
-    $suggestions[] = 'forums__containers__' . $variables['tid'];
+    $suggestions[] = 'forums__' . $tid;
+    $suggestions[] = 'forums__containers__' . $tid;
   }
   elseif (!$variables['forums'] && $variables['topics']) {
     $suggestions[] = 'forums__topics';
-    $suggestions[] = 'forums__' . $variables['tid'];
-    $suggestions[] = 'forums__topics__' . $variables['tid'];
+    $suggestions[] = 'forums__' . $tid;
+    $suggestions[] = 'forums__topics__' . $tid;
   }
   else {
-    $suggestions[] = 'forums__' . $variables['tid'];
+    $suggestions[] = 'forums__' . $tid;
   }
 
   return $suggestions;
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.routing.yml b/core/modules/system/tests/modules/theme_test/theme_test.routing.yml
index ef028f6..b4a7fd6 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.routing.yml
+++ b/core/modules/system/tests/modules/theme_test/theme_test.routing.yml
@@ -43,28 +43,28 @@ theme_test.request_listener:
     _access: 'TRUE'
 
 suggestion_alter:
-  pattern: '/theme-test/suggestion-alter'
+  path: '/theme-test/suggestion-alter'
   defaults:
     _content: '\Drupal\theme_test\ThemeTestController::suggestionAlter'
   requirements:
     _permission: 'access content'
 
 suggestion_provided:
-  pattern: '/theme-test/suggestion-provided'
+  path: '/theme-test/suggestion-provided'
   defaults:
     _content: '\Drupal\theme_test\ThemeTestController::suggestionProvided'
   requirements:
     _permission: 'access content'
 
 specific_suggestion_alter:
-  pattern: '/theme-test/specific-suggestion-alter'
+  path: '/theme-test/specific-suggestion-alter'
   defaults:
     _content: '\Drupal\theme_test\ThemeTestController::specificSuggestionAlter'
   requirements:
     _permission: 'access content'
 
 function_suggestion_alter:
-  pattern: '/theme-test/function-suggestion-alter'
+  path: '/theme-test/function-suggestion-alter'
   defaults:
     _content: '\Drupal\theme_test\ThemeTestController::functionSuggestionAlter'
   requirements: