diff --git a/core/lib/Drupal/Core/Theme/ThemeManager.php b/core/lib/Drupal/Core/Theme/ThemeManager.php
index f0119957d0..7cceb9d335 100644
--- a/core/lib/Drupal/Core/Theme/ThemeManager.php
+++ b/core/lib/Drupal/Core/Theme/ThemeManager.php
@@ -140,45 +140,50 @@ public function render($hook, array $variables) {
 
     $theme_registry = $this->themeRegistry->getRuntime();
 
-    // If an array of hook candidates were passed, use the first one that has an
-    // implementation.
-    if (is_array($hook)) {
-      foreach ($hook as $candidate) {
-        if ($theme_registry->has($candidate)) {
-          break;
-        }
-      }
-      $hook = $candidate;
+    // $hook is normally a string, but it can be an array. We only log error
+    // messages below if it was a string.
+    $is_hook_array = is_array($hook);
+
+    // While we search for templates, we create a full list of template
+    // suggestions that is later passed to theme_suggestions alter hooks.
+    $template_suggestions = $is_hook_array ? array_values($hook) : [$hook];
+
+    // The last element in our template suggestions gets special treatment.
+    // While the other elements must match exactly, the final element is
+    // expanded to create multiple possible matches by iteratively striping
+    // everything after the last '__' delimiter.
+    $last_hook = $suggestion = $is_hook_array ? $hook[array_key_last($hook)] : $hook;
+    while ($pos = strrpos($suggestion, '__')) {
+      $suggestion = substr($suggestion, 0, $pos);
+      $template_suggestions[] = $suggestion;
     }
-    // Save the original theme hook, so it can be supplied to theme variable
-    // preprocess callbacks.
-    $original_hook = $hook;
-
-    // If there's no implementation, check for more generic fallbacks.
-    // If there's still no implementation, log an error and return an empty
-    // string.
-    if (!$theme_registry->has($hook)) {
-      // Iteratively strip everything after the last '__' delimiter, until an
-      // implementation is found.
-      while ($pos = strrpos($hook, '__')) {
-        $hook = substr($hook, 0, $pos);
-        if ($theme_registry->has($hook)) {
-          break;
-        }
+
+    // Use the first hook candidate that has an implementation.
+    foreach ($template_suggestions as $candidate) {
+      if ($theme_registry->has($candidate)) {
+        // Save the original theme hook, so it can be supplied to theme variable
+        // preprocess callbacks.
+        $original_hook = $is_hook_array && in_array($candidate, $hook) ? $candidate : $last_hook;
+        $hook = $candidate;
+        $info = $theme_registry->get($hook);
+        break;
       }
-      if (!$theme_registry->has($hook)) {
-        // Only log a message when not trying theme suggestions ($hook being an
-        // array).
-        if (!isset($candidate)) {
-          \Drupal::logger('theme')->warning('Theme hook %hook not found.', ['%hook' => $hook]);
-        }
-        // There is no theme implementation for the hook passed. Return FALSE so
-        // the function calling
-        // \Drupal\Core\Theme\ThemeManagerInterface::render() can differentiate
-        // between a hook that exists and renders an empty string, and a hook
-        // that is not implemented.
-        return FALSE;
+    }
+
+    // If there's no implementation, log an error and return an empty string.
+    if (!isset($info)) {
+      // Only log a message if we #theme was a string. By default, all forms set
+      // #theme to an array containing the form ID and don't implement that as a
+      // theme hook, so we want to prevent errors for that common use case.
+      if (!$is_hook_array) {
+        \Drupal::logger('theme')->warning('Theme hook %hook not found.', ['%hook' => $candidate]);
       }
+      // There is no theme implementation for the hook passed. Return FALSE so
+      // the function calling
+      // \Drupal\Core\Theme\ThemeManagerInterface::render() can differentiate
+      // between a hook that exists and renders an empty string, and a hook
+      // that is not implemented.
+      return FALSE;
     }
 
     $info = $theme_registry->get($hook);
@@ -217,15 +222,17 @@ public function render($hook, array $variables) {
       'theme_hook_original' => $original_hook,
     ];
 
-    $suggestions = $this->buildThemeHookSuggestions($hook, $info['base hook'] ?? '', $variables);
+    $suggestions = $this->buildThemeHookSuggestions($hook, $info['base hook'] ?? '', $variables, $template_suggestions);
 
     // Check if each suggestion exists in the theme registry, and if so,
     // use it instead of the base hook. For example, a function may use
     // '#theme' => 'node', but a module can add 'node__article' as a suggestion
     // via hook_theme_suggestions_HOOK_alter(), enabling a theme to have
     // an alternate template file for article nodes.
+    $template_suggestion = $hook;
     foreach (array_reverse($suggestions) as $suggestion) {
       if ($theme_registry->has($suggestion)) {
+        $template_suggestion = $suggestion;
         $info = $theme_registry->get($suggestion);
         break;
       }
@@ -345,6 +352,10 @@ public function render($hook, array $variables) {
     if (isset($theme_hook_suggestion)) {
       $variables['theme_hook_suggestion'] = $theme_hook_suggestion;
     }
+    // Add two read-only variables that help the template engine understand
+    // how the template was chosen from among all suggestions.
+    $variables['template_suggestions'] = $template_suggestions;
+    $variables['template_suggestion'] = $template_suggestion;
     $output = $render_function($template_file, $variables);
     return ($output instanceof MarkupInterface) ? $output : (string) $output;
   }
@@ -368,20 +379,39 @@ public function render($hook, array $variables) {
    * @internal
    *   This method may change at any time. It is not for use outside this class.
    */
-  protected function buildThemeHookSuggestions(string $hook, string $info_base_hook, array &$variables): array {
+  protected function buildThemeHookSuggestions(string $hook, string $info_base_hook, array &$variables, array &$template_suggestions): array {
     // Set base hook for later use. For example if '#theme' => 'node__article'
     // is called, we run hook_theme_suggestions_node_alter() rather than
     // hook_theme_suggestions_node__article_alter(), and also pass in the base
     // hook as the last parameter to the suggestions alter hooks.
     $base_theme_hook = $info_base_hook ?: $hook;
 
+
+    // The $hook's theme registry may specify a "base hook" that differs from
+    // the base string of $hook. If so, we need to be aware of both strings.
+    $base_of_hook = explode('__', $hook)[0];
+
     // Invoke hook_theme_suggestions_HOOK().
     $suggestions = $this->moduleHandler->invokeAll('theme_suggestions_' . $base_theme_hook, [$variables]);
-    // If the theme implementation was invoked with a direct theme suggestion
-    // like '#theme' => 'node__article', add it to the suggestions array before
-    // invoking suggestion alter hooks.
-    if ($info_base_hook) {
-      $suggestions[] = $hook;
+
+    // Add all the template suggestions with the same base to the suggestions
+    // array before invoking suggestion alter hooks.
+    $contains_base_hook = in_array($base_theme_hook, $template_suggestions);
+    foreach (array_reverse($template_suggestions, TRUE) as $key => $suggestion) {
+      $suggestion_base = explode('__', $suggestion)[0];
+      if ($suggestion_base === $base_of_hook || $suggestion_base === $base_theme_hook) {
+        if ($suggestion !== $base_theme_hook) {
+          $suggestions[] = $suggestion;
+        }
+        // Temporarily remove from $template_suggestions the suggestions that we
+        // are adding to $suggestions given to the alter hooks. However, ensure
+        // that we leave one entry for the base hook, so we can splice those
+        // $suggestions back into $template_suggestions later.
+        if (($contains_base_hook && $suggestion !== $base_theme_hook)
+          || (!$contains_base_hook && $suggestion !== $hook)) {
+          unset($template_suggestions[$key]);
+        }
+      }
     }
 
     // Invoke hook_theme_suggestions_alter() and
@@ -393,6 +423,16 @@ protected function buildThemeHookSuggestions(string $hook, string $info_base_hoo
     $this->moduleHandler->alter($hooks, $suggestions, $variables, $base_theme_hook);
     $this->alter($hooks, $suggestions, $variables, $base_theme_hook);
 
+    // Merge $suggestions back into $template_suggestions before the "base hook"
+    // entry.
+    $template_suggestions = array_values($template_suggestions);
+    array_splice(
+      $template_suggestions,
+      array_search($contains_base_hook ? $base_theme_hook : $hook, $template_suggestions),
+      $contains_base_hook ? 0 : 1,
+      array_reverse($suggestions)
+    );
+
     return $suggestions;
   }
 
diff --git a/core/modules/system/tests/modules/theme_suggestions_base1_test/templates/theme-suggestions-base4-test-alternate--from-hook-theme-suggestions-hook-alter.html.twig b/core/modules/system/tests/modules/theme_suggestions_base1_test/templates/theme-suggestions-base4-test-alternate--from-hook-theme-suggestions-hook-alter.html.twig
new file mode 100644
index 0000000000..5adce85b9f
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_suggestions_base1_test/templates/theme-suggestions-base4-test-alternate--from-hook-theme-suggestions-hook-alter.html.twig
@@ -0,0 +1,2 @@
+{# Output for Theme API test #}
+This theme_suggestions_base4_test_alternate__from_hook_theme_suggestions_hook_alter template is implemented.
diff --git a/core/modules/system/tests/modules/theme_suggestions_base1_test/theme_suggestions_base1_test.info.yml b/core/modules/system/tests/modules/theme_suggestions_base1_test/theme_suggestions_base1_test.info.yml
new file mode 100644
index 0000000000..c9b249af3f
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_suggestions_base1_test/theme_suggestions_base1_test.info.yml
@@ -0,0 +1,5 @@
+name: 'Theme suggestions base1 test'
+type: module
+description: 'Support module for testing the theme_test_base1 base theme hook'
+package: Testing
+version: VERSION
diff --git a/core/modules/system/tests/modules/theme_suggestions_base1_test/theme_suggestions_base1_test.module b/core/modules/system/tests/modules/theme_suggestions_base1_test/theme_suggestions_base1_test.module
new file mode 100644
index 0000000000..323474f7f5
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_suggestions_base1_test/theme_suggestions_base1_test.module
@@ -0,0 +1,43 @@
+<?php
+
+/**
+ * @file
+ * Support module for testing theme suggestions.
+ */
+
+/**
+ * Implements hook_theme().
+ */
+function theme_suggestions_base1_test_theme($existing, $type, $theme, $path) {
+  $items['theme_suggestions_base4_test_alternate__from_hook_theme_suggestions_hook_alter'] = [
+    'variables' => [],
+  ];
+  return $items;
+}
+
+/**
+ * Implements hook_theme_suggestions_HOOK().
+ */
+function theme_suggestions_base1_test_theme_suggestions_theme_test_base1(array $variables) {
+  return [
+    'theme_test_base1__from_hook_theme_suggestions_hook',
+    'theme_test_base1__from_hook_theme_suggestions_hook_too',
+  ];
+}
+
+/**
+ * Implements hook_theme_suggestions_HOOK_alter().
+ */
+function theme_suggestions_base1_test_theme_suggestions_theme_test_base1_alter(array &$suggestions, array $variables, $hook) {
+  $suggestions[] = $hook . '__from_hook_theme_suggestions_hook_alter__but_reordered';
+
+  // We move a suggestion from hook_theme_suggestions_HOOK() to come after our
+  // first suggestion above. We also create an intentional gap in the numeric
+  // keys using this common method; Drupal core should handle non-contiguous
+  // index keys.
+  $moved_suggestion = $hook . '__from_hook_theme_suggestions_hook_too';
+  unset($suggestions[array_search($moved_suggestion, $suggestions)]);
+  $suggestions[] = $moved_suggestion;
+
+  $suggestions[] = $hook . '__from_hook_theme_suggestions_hook_alter';
+}
diff --git a/core/modules/system/tests/modules/theme_suggestions_base2_ignored_test/theme_suggestions_base2_ignored_test.info.yml b/core/modules/system/tests/modules/theme_suggestions_base2_ignored_test/theme_suggestions_base2_ignored_test.info.yml
new file mode 100644
index 0000000000..3f2ad1d267
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_suggestions_base2_ignored_test/theme_suggestions_base2_ignored_test.info.yml
@@ -0,0 +1,5 @@
+name: 'Theme suggestions base2 ignored test'
+type: module
+description: 'Support module for testing the ignored theme_test_base2 base theme hook'
+package: Testing
+version: VERSION
diff --git a/core/modules/system/tests/modules/theme_suggestions_base2_ignored_test/theme_suggestions_base2_ignored_test.module b/core/modules/system/tests/modules/theme_suggestions_base2_ignored_test/theme_suggestions_base2_ignored_test.module
new file mode 100644
index 0000000000..3b47bdc93e
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_suggestions_base2_ignored_test/theme_suggestions_base2_ignored_test.module
@@ -0,0 +1,15 @@
+<?php
+
+/**
+ * @file
+ * Support module for testing theme suggestions.
+ */
+
+/**
+ * Implements hook_theme_suggestions_HOOK_alter().
+ */
+function theme_suggestions_base2_ignored_test_theme_suggestions_theme_test_base1_alter(array &$suggestions, array $variables, $hook) {
+  // Add a suggestion just to confirm that the theme_test_base2 suggestion is
+  // normally ignored.
+  $suggestions[] = 'theme_test_base2';
+}
diff --git a/core/modules/system/tests/modules/theme_suggestions_base2_test/templates/theme-test-base2--from-hook-theme-suggestions-hook.html.twig b/core/modules/system/tests/modules/theme_suggestions_base2_test/templates/theme-test-base2--from-hook-theme-suggestions-hook.html.twig
new file mode 100644
index 0000000000..17ef2b27d8
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_suggestions_base2_test/templates/theme-test-base2--from-hook-theme-suggestions-hook.html.twig
@@ -0,0 +1,2 @@
+{# Output for Theme API test #}
+This theme_test_base2__from_hook_theme_suggestions_hook template is implemented, but never used.
diff --git a/core/modules/system/tests/modules/theme_suggestions_base2_test/templates/theme-test-base2--from-theme-property--without-base.html.twig b/core/modules/system/tests/modules/theme_suggestions_base2_test/templates/theme-test-base2--from-theme-property--without-base.html.twig
new file mode 100644
index 0000000000..4b6a0a11cb
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_suggestions_base2_test/templates/theme-test-base2--from-theme-property--without-base.html.twig
@@ -0,0 +1,2 @@
+{# Output for Theme API test #}
+This theme_test_base2__from_theme_property__without_base template is implemented.
diff --git a/core/modules/system/tests/modules/theme_suggestions_base2_test/theme_suggestions_base2_test.info.yml b/core/modules/system/tests/modules/theme_suggestions_base2_test/theme_suggestions_base2_test.info.yml
new file mode 100644
index 0000000000..4389d136de
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_suggestions_base2_test/theme_suggestions_base2_test.info.yml
@@ -0,0 +1,5 @@
+name: 'Theme suggestions base2 test'
+type: module
+description: 'Support module for testing the theme_test_base2 base theme hook'
+package: Testing
+version: VERSION
diff --git a/core/modules/system/tests/modules/theme_suggestions_base2_test/theme_suggestions_base2_test.module b/core/modules/system/tests/modules/theme_suggestions_base2_test/theme_suggestions_base2_test.module
new file mode 100644
index 0000000000..4a77f9b30b
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_suggestions_base2_test/theme_suggestions_base2_test.module
@@ -0,0 +1,39 @@
+<?php
+
+/**
+ * @file
+ * Support module for testing theme suggestions.
+ */
+
+/**
+ * Implements hook_theme().
+ */
+function theme_suggestions_base2_test_theme($existing, $type, $theme, $path) {
+  $items['theme_test_base2__from_theme_property__without_base'] = [
+    'base hook' => 'theme_test_base2',
+    'variables' => [],
+  ];
+  $items['theme_test_base2__from_hook_theme_suggestions_hook'] = [
+    'base hook' => 'theme_test_base2',
+    'variables' => [],
+  ];
+  return $items;
+}
+
+/**
+ * Implements hook_theme_suggestions_HOOK().
+ */
+function theme_suggestions_base2_test_theme_suggestions_theme_test_base2(array $variables) {
+  return [
+    'theme_test_base2__from_hook_theme_suggestions_hook',
+  ];
+}
+
+/**
+ * Implements hook_theme_suggestions_HOOK_alter().
+ */
+function theme_suggestions_base2_test_theme_suggestions_theme_test_base2_alter(array &$suggestions, array $variables, $hook) {
+  // Add a suggestion just to confirm that the theme_test_base2 suggestion is
+  // normally ignored.
+  $suggestions[] = 'theme_test_base2__from_hook_theme_suggestions_hook_alter';
+}
diff --git a/core/modules/system/tests/modules/theme_suggestions_base3_test/templates/theme-test-base3.html.twig b/core/modules/system/tests/modules/theme_suggestions_base3_test/templates/theme-test-base3.html.twig
new file mode 100644
index 0000000000..287557002e
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_suggestions_base3_test/templates/theme-test-base3.html.twig
@@ -0,0 +1,2 @@
+{# Output for Theme API test #}
+This theme_test_base3 template is implemented.
diff --git a/core/modules/system/tests/modules/theme_suggestions_base3_test/theme_suggestions_base3_test.info.yml b/core/modules/system/tests/modules/theme_suggestions_base3_test/theme_suggestions_base3_test.info.yml
new file mode 100644
index 0000000000..38302bf332
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_suggestions_base3_test/theme_suggestions_base3_test.info.yml
@@ -0,0 +1,5 @@
+name: 'Theme suggestions base3 test'
+type: module
+description: 'Support module for testing the theme_test_base3 base theme hook'
+package: Testing
+version: VERSION
diff --git a/core/modules/system/tests/modules/theme_suggestions_base3_test/theme_suggestions_base3_test.module b/core/modules/system/tests/modules/theme_suggestions_base3_test/theme_suggestions_base3_test.module
new file mode 100644
index 0000000000..c8cccc9fe7
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_suggestions_base3_test/theme_suggestions_base3_test.module
@@ -0,0 +1,27 @@
+<?php
+
+/**
+ * @file
+ * Support module for testing theme suggestions.
+ */
+
+/**
+ * Implements hook_theme().
+ */
+function theme_suggestions_base3_test_theme($existing, $type, $theme, $path) {
+  $items['theme_test_base3'] = [
+    'variables' => [],
+  ];
+  return $items;
+}
+
+/**
+ * Implements hook_theme_suggestions_HOOK_alter().
+ */
+function theme_suggestions_base3_test_theme_suggestions_theme_test_base3_alter(array &$suggestions, array $variables, $hook) {
+  $from_theme_array = array_search('theme_test_base3__from_theme_property', $suggestions);
+  if ($from_theme_array !== FALSE) {
+    $suggestions[$from_theme_array] .= '__seen_by_alter';
+  }
+  $suggestions[] = 'theme_test_base3__from_hook_theme_suggestions_hook_alter';
+}
diff --git a/core/modules/system/tests/modules/theme_suggestions_base4_test/templates/theme-test-base4.html.twig b/core/modules/system/tests/modules/theme_suggestions_base4_test/templates/theme-test-base4.html.twig
new file mode 100644
index 0000000000..78bc128f0a
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_suggestions_base4_test/templates/theme-test-base4.html.twig
@@ -0,0 +1,2 @@
+{# Output for Theme API test #}
+This theme_test_base4 template is implemented and has a different "base hook".
diff --git a/core/modules/system/tests/modules/theme_suggestions_base4_test/theme_suggestions_base4_test.info.yml b/core/modules/system/tests/modules/theme_suggestions_base4_test/theme_suggestions_base4_test.info.yml
new file mode 100644
index 0000000000..f5e876d495
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_suggestions_base4_test/theme_suggestions_base4_test.info.yml
@@ -0,0 +1,5 @@
+name: 'Theme suggestions base4 test'
+type: module
+description: 'Support module for testing the theme_test_base4 base theme hook'
+package: Testing
+version: VERSION
diff --git a/core/modules/system/tests/modules/theme_suggestions_base4_test/theme_suggestions_base4_test.module b/core/modules/system/tests/modules/theme_suggestions_base4_test/theme_suggestions_base4_test.module
new file mode 100644
index 0000000000..9db0eaee7f
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_suggestions_base4_test/theme_suggestions_base4_test.module
@@ -0,0 +1,33 @@
+<?php
+
+/**
+ * @file
+ * Support module for testing theme suggestions.
+ */
+
+/**
+ * Implements hook_theme().
+ */
+function theme_suggestions_base4_test_theme($existing, $type, $theme, $path) {
+  $items['theme_test_base4'] = [
+    'base hook' => 'theme_suggestions_base4_test_alternate',
+    'variables' => [],
+  ];
+  return $items;
+}
+
+/**
+ * Implements hook_theme_suggestions_HOOK().
+ */
+function theme_suggestions_base4_test_theme_suggestions_theme_suggestions_base4_test_alternate(array $variables) {
+  return [
+    'theme_suggestions_base4_test_alternate__from_hook_theme_suggestions_hook',
+  ];
+}
+
+/**
+ * Implements hook_theme_suggestions_HOOK_alter().
+ */
+function theme_suggestions_base4_test_theme_suggestions_theme_suggestions_base4_test_alternate_alter(array &$suggestions, array $variables, $hook) {
+  $suggestions[] = $hook . '__from_hook_theme_suggestions_hook_alter';
+}
diff --git a/core/modules/system/tests/themes/test_theme/templates/theme-test-general-suggestions--module-override.html.twig b/core/modules/system/tests/modules/theme_suggestions_test/templates/theme-test-general-suggestions--module-override.html.twig
similarity index 100%
rename from core/modules/system/tests/themes/test_theme/templates/theme-test-general-suggestions--module-override.html.twig
rename to core/modules/system/tests/modules/theme_suggestions_test/templates/theme-test-general-suggestions--module-override.html.twig
diff --git a/core/modules/system/tests/themes/test_theme/templates/theme-test-suggestions--module-override.html.twig b/core/modules/system/tests/modules/theme_suggestions_test/templates/theme-test-suggestions--module-override.html.twig
similarity index 100%
rename from core/modules/system/tests/themes/test_theme/templates/theme-test-suggestions--module-override.html.twig
rename to core/modules/system/tests/modules/theme_suggestions_test/templates/theme-test-suggestions--module-override.html.twig
diff --git a/core/modules/system/tests/modules/theme_suggestions_test/theme_suggestions_test.module b/core/modules/system/tests/modules/theme_suggestions_test/theme_suggestions_test.module
index 789f8f2e00..45438ecef1 100644
--- a/core/modules/system/tests/modules/theme_suggestions_test/theme_suggestions_test.module
+++ b/core/modules/system/tests/modules/theme_suggestions_test/theme_suggestions_test.module
@@ -5,6 +5,21 @@
  * Support module for testing theme suggestions.
  */
 
+/**
+ * Implements hook_theme().
+ */
+function theme_suggestions_test_theme($existing, $type, $theme, $path) {
+  $items['theme_test_general_suggestions__module_override'] = [
+    'base hook' => 'theme_test_general_suggestions',
+    'variables' => [],
+  ];
+  $items['theme_test_suggestions__module_override'] = [
+    'base hook' => 'theme_test_suggestions',
+    'variables' => [],
+  ];
+  return $items;
+}
+
 /**
  * Implements hook_theme_suggestions_alter().
  */
@@ -19,14 +34,14 @@ function theme_suggestions_test_theme_suggestions_alter(array &$suggestions, arr
 /**
  * Implements hook_theme_suggestions_HOOK_alter().
  */
-function theme_suggestions_test_theme_suggestions_theme_test_suggestions_alter(array &$suggestions, array $variables) {
+function theme_suggestions_test_theme_suggestions_theme_test_suggestions_alter(array &$suggestions, array $variables, $hook) {
   \Drupal::messenger()->addStatus(__FUNCTION__ . '() executed.');
-  $suggestions[] = 'theme_test_suggestions__module_override';
+  $suggestions[] = $hook . '__module_override';
 }
 
 /**
  * Implements hook_theme_suggestions_HOOK_alter().
  */
-function theme_suggestions_test_theme_suggestions_theme_test_specific_suggestions_alter(array &$suggestions, array $variables) {
-  $suggestions[] = 'theme_test_specific_suggestions__variant__foo';
+function theme_suggestions_test_theme_suggestions_theme_test_specific_suggestions_alter(array &$suggestions, array $variables, $hook) {
+  $suggestions[] = $hook . '__variant__foo';
 }
diff --git a/core/modules/system/tests/modules/theme_test/src/ThemeTestController.php b/core/modules/system/tests/modules/theme_test/src/ThemeTestController.php
index 5d8badabeb..89c66f8203 100644
--- a/core/modules/system/tests/modules/theme_test/src/ThemeTestController.php
+++ b/core/modules/system/tests/modules/theme_test/src/ThemeTestController.php
@@ -72,34 +72,6 @@ public function testRequestListener() {
     return ['#markup' => $GLOBALS['theme_test_output']];
   }
 
-  /**
-   * Menu callback for testing suggestion alter hooks with template files.
-   */
-  public function suggestionProvided() {
-    return ['#theme' => 'theme_test_suggestion_provided'];
-  }
-
-  /**
-   * Menu callback for testing suggestion alter hooks with template files.
-   */
-  public function suggestionAlter() {
-    return ['#theme' => 'theme_test_suggestions'];
-  }
-
-  /**
-   * Menu callback for testing hook_theme_suggestions_alter().
-   */
-  public function generalSuggestionAlter() {
-    return ['#theme' => 'theme_test_general_suggestions'];
-  }
-
-  /**
-   * Menu callback for testing suggestion alter hooks with specific suggestions.
-   */
-  public function specificSuggestionAlter() {
-    return ['#theme' => 'theme_test_specific_suggestions__variant'];
-  }
-
   /**
    * Controller to ensure that no theme is initialized.
    *
diff --git a/core/modules/system/tests/modules/theme_test/templates/theme-test-base1.html.twig b/core/modules/system/tests/modules/theme_test/templates/theme-test-base1.html.twig
new file mode 100644
index 0000000000..2bea8c84b9
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_test/templates/theme-test-base1.html.twig
@@ -0,0 +1,2 @@
+{# Output for Theme API test #}
+Template for testing suggestion hooks when #theme contains a list of theme suggestions.
diff --git a/core/modules/system/tests/modules/theme_test/templates/theme-test-base2.html.twig b/core/modules/system/tests/modules/theme_test/templates/theme-test-base2.html.twig
new file mode 100644
index 0000000000..e157f28aad
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_test/templates/theme-test-base2.html.twig
@@ -0,0 +1,2 @@
+{# Output for Theme API test #}
+This theme_test_base2 template is implemented, but never used.
diff --git a/core/modules/system/tests/modules/theme_test/templates/theme-test-xss-suggestion.html.twig b/core/modules/system/tests/modules/theme_test/templates/theme-test-xss-suggestion.html.twig
new file mode 100644
index 0000000000..a8e58846be
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_test/templates/theme-test-xss-suggestion.html.twig
@@ -0,0 +1,2 @@
+{# Output for Theme API test #}
+Template for testing XSS in theme hook suggestions.
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.module b/core/modules/system/tests/modules/theme_test/theme_test.module
index dc0f07c6c9..59639203fc 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -18,9 +18,11 @@ function theme_test_theme($existing, $type, $theme, $path) {
   ];
   $items['theme_test_template_test'] = [
     'template' => 'theme_test.template_test',
+    'variables' => [],
   ];
   $items['theme_test_template_test_2'] = [
     'template' => 'theme_test.template_test',
+    'variables' => [],
   ];
   $items['theme_test_suggestion_provided'] = [
     'variables' => [],
@@ -34,6 +36,15 @@ function theme_test_theme($existing, $type, $theme, $path) {
   $items['theme_test_general_suggestions'] = [
     'variables' => ['module_hook' => 'theme_test_theme', 'theme_hook' => 'none'],
   ];
+  $items['theme_test_base1'] = [
+    'variables' => [],
+  ];
+  $items['theme_test_base2'] = [
+    'variables' => [],
+  ];
+  $items['theme_test_xss_suggestion'] = [
+    'variables' => [],
+  ];
   $items['theme_test_foo'] = [
     'variables' => ['foo' => NULL],
   ];
@@ -158,11 +169,20 @@ function theme_test_theme_suggestions_theme_test_suggestion_provided(array $vari
   return ['theme_test_suggestion_provided__foo'];
 }
 
+/**
+ * Implements hook_theme_suggestions_HOOK().
+ */
+function theme_test_theme_suggestions_theme_test_suggestions(array $variables) {
+  \Drupal::messenger()->addStatus(__FUNCTION__ . '() executed.');
+}
+
 /**
  * Implements hook_theme_suggestions_alter().
  */
 function theme_test_theme_suggestions_alter(array &$suggestions, array $variables, $hook) {
-  \Drupal::messenger()->addStatus(__FUNCTION__ . '() executed for ' . $hook . '.');
+  if ($hook === 'theme_test_suggestions') {
+    \Drupal::messenger()->addStatus(__FUNCTION__ . '() executed.');
+  }
 }
 
 /**
@@ -188,9 +208,9 @@ function theme_test_system_info_alter(array &$info, Extension $file, $type) {
 /**
  * Implements hook_theme_suggestions_HOOK().
  */
-function theme_test_theme_suggestions_node(array $variables) {
+function theme_test_theme_suggestions_theme_test_xss_suggestion(array $variables) {
   $xss = '<script type="text/javascript">alert(\'yo\');</script>';
-  $suggestions[] = 'node__' . $xss;
+  $suggestions[] = 'theme_test_xss_suggestion__' . $xss;
 
   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 1299fe9562..9ee3c31804 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
@@ -39,34 +39,6 @@ theme_test.request_listener:
   requirements:
     _access: 'TRUE'
 
-suggestion_alter:
-  path: '/theme-test/suggestion-alter'
-  defaults:
-    _controller: '\Drupal\theme_test\ThemeTestController::suggestionAlter'
-  requirements:
-    _access: 'TRUE'
-
-theme_test.general_suggestion_alter:
-  path: '/theme-test/general-suggestion-alter'
-  defaults:
-    _controller: '\Drupal\theme_test\ThemeTestController::generalSuggestionAlter'
-  requirements:
-    _access: 'TRUE'
-
-suggestion_provided:
-  path: '/theme-test/suggestion-provided'
-  defaults:
-    _controller: '\Drupal\theme_test\ThemeTestController::suggestionProvided'
-  requirements:
-    _access: 'TRUE'
-
-specific_suggestion_alter:
-  path: '/theme-test/specific-suggestion-alter'
-  defaults:
-    _controller: '\Drupal\theme_test\ThemeTestController::specificSuggestionAlter'
-  requirements:
-    _access: 'TRUE'
-
 theme_test.non_html:
   path: '/theme-test/non-html'
   defaults:
diff --git a/core/modules/system/tests/src/Kernel/Theme/ThemeSuggestionsAlterTest.php b/core/modules/system/tests/src/Kernel/Theme/ThemeSuggestionsAlterTest.php
new file mode 100644
index 0000000000..9b7d39c855
--- /dev/null
+++ b/core/modules/system/tests/src/Kernel/Theme/ThemeSuggestionsAlterTest.php
@@ -0,0 +1,711 @@
+<?php
+
+namespace Drupal\Tests\system\Kernel\Theme;
+
+use Drupal\Core\DependencyInjection\ContainerBuilder;
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * Tests theme suggestions alter hooks.
+ *
+ * @group Theme
+ */
+class ThemeSuggestionsAlterTest extends KernelTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['theme_test', 'system'];
+
+  /**
+   * {@inheritdoc}
+   */
+  public function register(ContainerBuilder $container) {
+    parent::register($container);
+    // Enable Twig debugging.
+    $parameters = $container->getParameter('twig.config');
+    $parameters['debug'] = TRUE;
+    $container->setParameter('twig.config', $parameters);
+  }
+
+  /**
+   * {@inheritdoc}
+   *
+   * We override KernelTestBase::render() so that it outputs Twig debug comments
+   * only for the render array given in a test and not for an entire page.
+   */
+  protected function render(array &$elements): string {
+    return $this->container->get('renderer')->renderRoot($elements);
+  }
+
+  /**
+   * Helper function to test render arrays with modules and themes.
+   *
+   * @param array $build
+   *   A render array.
+   * @param string[] $modules
+   *   An array of modules to install before the test.
+   * @param string $theme
+   *   The theme to set as the default theme.
+   * @param string[] $expected
+   *   The string(s) expected in the rendered output.
+   * @param string[] $unexpected
+   *   The string(s) expected to NOT occur in the rendered output.
+   *
+   * @throws \Exception
+   */
+  public function runTemplateSuggestionTest(array $build, array $modules, string $theme, array $expected, array $unexpected = []) {
+    // Enable modules.
+    if (!empty($modules)) {
+      $this->enableModules($modules);
+    }
+    // Set the default theme.
+    if ($theme) {
+      $this->container->get('theme_installer')->install([$theme]);
+      $this->config('system.theme')
+        ->set('default', $theme)
+        ->save();
+    }
+
+    // Render a template.
+    $output = $this->render($build);
+
+    // Check the output for expected results.
+    foreach ($expected as $expected_string) {
+      $this->assertStringContainsString($expected_string, $output, $this->getName());
+    }
+
+    // Check the output for unexpected results.
+    foreach ($unexpected as $unexpected_string) {
+      $this->assertStringNotContainsString($unexpected_string, $output, $this->getName());
+    }
+  }
+
+  /**
+   * Tests hook_theme_suggestions_HOOK.
+   *
+   * Note: themes cannot use this hook.
+   *
+   * @param string[] $modules
+   *   An array of modules to install before the test.
+   * @param string $theme
+   *   The theme to set as the default theme.
+   * @param string[] $expected
+   *   The string(s) expected in the rendered output.
+   *
+   * @dataProvider providerHookThemeSuggestionsHook
+   *
+   * @throws \Exception
+   */
+  public function testHookThemeSuggestionsHook(array $modules, string $theme, array $expected) {
+    $this->runTemplateSuggestionTest(
+      [
+        '#theme' => 'theme_test_suggestion_provided',
+      ],
+      $modules,
+      $theme,
+      $expected
+    );
+  }
+
+  /**
+   * Data provider for testHookThemeSuggestionsHook().
+   *
+   * @see testHookThemeSuggestionsHook()
+   */
+  public function providerHookThemeSuggestionsHook(): array {
+    return [
+      'Base template used when suggestion template not found' => [
+        'modules' => [],
+        'theme' => '',
+        'expected' => [
+          'Template for testing suggestions provided by the module declaring the theme hook.',
+        ],
+      ],
+      'Suggestion template used when suggestion template is found' => [
+        'modules' => [],
+        // The test_theme contains a template suggested by theme_test.module in
+        // theme_test_theme_suggestions_theme_test_suggestion_provided().
+        'theme' => 'test_theme',
+        'expected' => [
+          'Template overridden based on suggestion provided by the module declaring the theme hook.',
+        ],
+      ],
+    ];
+  }
+
+  /**
+   * Tests hook_theme_suggestions_alter().
+   *
+   * @param string[] $modules
+   *   An array of modules to install before the test.
+   * @param string $theme
+   *   The theme to set as the default theme.
+   * @param string[] $expected
+   *   The string(s) expected in the rendered output.
+   *
+   * @dataProvider providerHookThemeSuggestionsAlter
+   *
+   * @throws \Exception
+   */
+  public function testHookThemeSuggestionsAlter(array $modules, string $theme, array $expected) {
+    $this->runTemplateSuggestionTest(
+      [
+        '#theme' => 'theme_test_general_suggestions',
+      ],
+      $modules,
+      $theme,
+      $expected
+    );
+  }
+
+  /**
+   * Data provider for testHookThemeSuggestionsAlter().
+   *
+   * @see testHookThemeSuggestionsAlter()
+   */
+  public function providerHookThemeSuggestionsAlter(): array {
+    $extension = '.html.twig';
+    return [
+      'Base template used when suggestion template is not available' => [
+        'modules' => [],
+        'theme' => '',
+        'expected' => [
+          'Original template for testing hook_theme_suggestions_alter().',
+          "<!-- BEGIN OUTPUT from 'core/modules/system/tests/modules/theme_test/templates/theme-test-general-suggestions$extension' -->",
+        ],
+      ],
+      'Suggestion provided by a module\'s hook_theme_suggestions_alter is used' => [
+        // @see theme_suggestions_test_theme_suggestions_alter()
+        'modules' => ['theme_suggestions_test'],
+        'theme' => '',
+        'expected' => [
+          'Template overridden based on new theme suggestion provided by a module via hook_theme_suggestions_alter().',
+          "<!-- BEGIN OUTPUT from 'core/modules/system/tests/modules/theme_suggestions_test/templates/theme-test-general-suggestions--module-override$extension' -->",
+        ],
+      ],
+      'Suggestion provided by a theme\'s hook_theme_suggestions_alter is used' => [
+        'modules' => [],
+        // @see test_theme_theme_suggestions_alter()
+        'theme' => 'test_theme',
+        'expected' => [
+          'Template overridden based on new theme suggestion provided by the test_theme theme via hook_theme_suggestions_alter().',
+          "<!-- BEGIN OUTPUT from 'core/modules/system/tests/themes/test_theme/templates/theme-test-general-suggestions--theme-override$extension' -->",
+        ],
+      ],
+      'Themes implementing hook_theme_suggestions_alter override modules' => [
+        'modules' => ['theme_suggestions_test'],
+        'theme' => 'test_theme',
+        'expected' => [
+          'Template overridden based on new theme suggestion provided by the test_theme theme via hook_theme_suggestions_alter().',
+          "<!-- BEGIN OUTPUT from 'core/modules/system/tests/themes/test_theme/templates/theme-test-general-suggestions--theme-override$extension' -->",
+        ],
+        'unexpected' => [
+          'Template overridden based on new theme suggestion provided by a module via hook_theme_suggestions_alter().',
+        ],
+      ],
+    ];
+  }
+
+  /**
+   * Tests hook_theme_suggestions_HOOK_alter().
+   *
+   * @param string[] $modules
+   *   An array of modules to install before the test.
+   * @param string $theme
+   *   The theme to set as the default theme.
+   * @param string[] $expected
+   *   The string(s) expected in the rendered output.
+   *
+   * @dataProvider providerHookThemeSuggestionsHookAlter
+   *
+   * @throws \Exception
+   */
+  public function testHookThemeSuggestionsHookAlter(array $modules, string $theme, array $expected) {
+    $this->runTemplateSuggestionTest(
+      [
+        '#theme' => 'theme_test_suggestions',
+      ],
+      $modules,
+      $theme,
+      $expected
+    );
+  }
+
+  /**
+   * Data provider for testHookThemeSuggestionsHookAlter().
+   *
+   * @see testHookThemeSuggestionsHookAlter()
+   */
+  public function providerHookThemeSuggestionsHookAlter(): array {
+    return [
+      'Base template used when suggestion template is not available' => [
+        'modules' => [],
+        'theme' => '',
+        'expected' => [
+          'Original template for testing hook_theme_suggestions_HOOK_alter().',
+        ],
+      ],
+      'Suggestion provided by a module\'s hook_theme_suggestions_HOOK_alter is used' => [
+        // @see theme_suggestions_test_theme_suggestions_theme_test_suggestions_alter()
+        'modules' => ['theme_suggestions_test'],
+        'theme' => '',
+        'expected' => [
+          'Template overridden based on new theme suggestion provided by a module via hook_theme_suggestions_HOOK_alter().',
+        ],
+      ],
+      'Suggestion provided by a theme\'s hook_theme_suggestions_HOOK_alter is used' => [
+        'modules' => [],
+        // @see test_theme_theme_suggestions_theme_test_suggestions_alter()
+        'theme' => 'test_theme',
+        'expected' => [
+          'Template overridden based on new theme suggestion provided by the test_theme theme via hook_theme_suggestions_HOOK_alter().',
+        ],
+      ],
+    ];
+  }
+
+  /**
+   * Tests non-"base hook" suggestions with hook_theme_suggestions_HOOK_alter().
+   *
+   * @param string[] $modules
+   *   An array of modules to install before the test.
+   * @param string $theme
+   *   The theme to set as the default theme.
+   * @param string[] $expected
+   *   The string(s) expected in the rendered output.
+   *
+   * @dataProvider providerNonBaseHookThemeSuggestions
+   *
+   * @throws \Exception
+   */
+  public function testNonBaseHookThemeSuggestions(array $modules, string $theme, array $expected) {
+    $this->runTemplateSuggestionTest(
+      [
+        '#theme' => 'theme_test_specific_suggestions__variant',
+      ],
+      $modules,
+      $theme,
+      $expected
+    );
+  }
+
+  /**
+   * Data provider for testNonBaseHookThemeSuggestions().
+   *
+   * @see testNonBaseHookThemeSuggestions()
+   */
+  public function providerNonBaseHookThemeSuggestions(): array {
+    $extension = '.html.twig';
+    return [
+      'Base template used when suggestion template is not available' => [
+        'modules' => [],
+        'theme' => '',
+        'expected' => [
+          'Template for testing specific theme calls.',
+        ],
+      ],
+      'Suggestion provided by a module\'s hook_theme_suggestions_HOOK_alter is used' => [
+        // @see theme_suggestions_test_theme_suggestions_theme_test_specific_suggestions_alter()
+        'modules' => ['theme_suggestions_test'],
+        'theme' => 'test_theme',
+        'expected' => [
+          'Template overridden based on suggestion alter hook determined by a module\'s hook_theme_suggestions_HOOK_alter().',
+          '<!-- FILE NAME SUGGESTIONS:' . PHP_EOL
+          . '   x theme-test-specific-suggestions--variant--foo' . $extension . PHP_EOL
+          . '   * theme-test-specific-suggestions--variant' . $extension . PHP_EOL
+          . '   * theme-test-specific-suggestions' . $extension . PHP_EOL
+          . '-->' . PHP_EOL,
+        ],
+      ],
+      'Suggestion template provided by a theme' => [
+        'modules' => [],
+        'theme' => 'test_theme',
+        'expected' => [
+          'Template overridden based on suggestion alter hook determined by the base hook.',
+          '<!-- FILE NAME SUGGESTIONS:' . PHP_EOL
+          . '   x theme-test-specific-suggestions--variant' . $extension . PHP_EOL
+          . '   * theme-test-specific-suggestions' . $extension . PHP_EOL
+          . '-->' . PHP_EOL,
+        ],
+      ],
+    ];
+  }
+
+  /**
+   * Tests debug markup for non-"base hook" suggestions without implementation.
+   *
+   * @throws \Exception
+   */
+  public function testUnimplementedNonBaseHookThemeSuggestions() {
+    $extension = '.html.twig';
+    $this->runTemplateSuggestionTest(
+      [
+        '#theme' => 'theme_test_specific_suggestions__variant_not_found__too',
+      ],
+      [],
+      '',
+      [
+        "THEME HOOK: 'theme_test_specific_suggestions__variant_not_found__too'",
+        '<!-- FILE NAME SUGGESTIONS:' . PHP_EOL
+        . '   * theme-test-specific-suggestions--variant-not-found--too' . $extension . PHP_EOL
+        . '   * theme-test-specific-suggestions--variant-not-found' . $extension . PHP_EOL
+        . '   x theme-test-specific-suggestions' . $extension . PHP_EOL
+        . '-->',
+      ]
+    );
+  }
+
+  /**
+   * Tests an array of suggestions with all alter hooks.
+   *
+   * @param string[] $modules
+   *   An array of modules to install before the test.
+   * @param string $theme
+   *   The theme to set as the default theme.
+   * @param string[] $expected
+   *   The string(s) expected in the rendered output.
+   * @param string[] $unexpected
+   *   The string(s) expected to NOT occur in the rendered output.
+   *
+   * @dataProvider providerThemeSuggestionsOrdering
+   *
+   * @throws \Exception
+   */
+  public function testThemeSuggestionsOrdering(array $modules, string $theme, array $expected, array $unexpected) {
+    $this->runTemplateSuggestionTest(
+      [
+        '#theme' => 'theme_test_base1__from_theme_property__too',
+      ],
+      $modules,
+      $theme,
+      $expected,
+      $unexpected
+    );
+  }
+
+  /**
+   * Data provider for testThemeSuggestionsOrdering().
+   *
+   * @see testThemeSuggestionsOrdering()
+   */
+  public function providerThemeSuggestionsOrdering(): array {
+    $extension = '.html.twig';
+    return [
+      '#theme property suggestions always override ones from hook_theme_suggestions_hook' => [
+        'modules' => ['theme_suggestions_base1_test'],
+        'theme' => '',
+        'expected' => [
+          'Template for testing suggestion hooks when #theme contains a list of theme suggestions.',
+          '<!-- FILE NAME SUGGESTIONS:' . PHP_EOL
+          . '   * theme-test-base1--from-hook-theme-suggestions-hook-alter' . $extension . PHP_EOL
+          . '   * theme-test-base1--from-hook-theme-suggestions-hook-too' . $extension . PHP_EOL
+          . '   * theme-test-base1--from-hook-theme-suggestions-hook-alter--but-reordered' . $extension . PHP_EOL
+          . '   * theme-test-base1--from-theme-property--too' . $extension . PHP_EOL
+          . '   * theme-test-base1--from-theme-property' . $extension . PHP_EOL
+          . '   * theme-test-base1--from-hook-theme-suggestions-hook' . $extension . PHP_EOL
+          . '   x theme-test-base1' . $extension . PHP_EOL
+          . '-->' . PHP_EOL,
+        ],
+        'unexpected' => [],
+      ],
+      'adding a new template implementation does not change order of suggestions' => [
+        'modules' => ['theme_suggestions_base1_test'],
+        'theme' => 'test_theme',
+        'expected' => [
+          'This theme_test_base1__from_theme_property__too template is implemented.',
+          '<!-- FILE NAME SUGGESTIONS:' . PHP_EOL
+          . '   * theme-test-base1--from-hook-theme-suggestions-hook-alter' . $extension . PHP_EOL
+          . '   * theme-test-base1--from-hook-theme-suggestions-hook-too' . $extension . PHP_EOL
+          . '   * theme-test-base1--from-hook-theme-suggestions-hook-alter--but-reordered' . $extension . PHP_EOL
+          . '   x theme-test-base1--from-theme-property--too' . $extension . PHP_EOL
+          . '   * theme-test-base1--from-theme-property' . $extension . PHP_EOL
+          . '   * theme-test-base1--from-hook-theme-suggestions-hook' . $extension . PHP_EOL
+          . '   * theme-test-base1' . $extension . PHP_EOL
+          . '-->' . PHP_EOL,
+        ],
+        'unexpected' => [],
+      ],
+    ];
+  }
+
+  /**
+   * Tests an array of suggestions with all alter hooks.
+   *
+   * @param string[] $modules
+   *   An array of modules to install before the test.
+   * @param string $theme
+   *   The theme to set as the default theme.
+   * @param string[] $expected
+   *   The string(s) expected in the rendered output.
+   * @param string[] $unexpected
+   *   The string(s) expected to NOT occur in the rendered output.
+   *
+   * @dataProvider providerArrayThemeSuggestions
+   *
+   * @throws \Exception
+   */
+  public function testArrayThemeSuggestions(array $modules, string $theme, array $expected, array $unexpected) {
+    $this->runTemplateSuggestionTest(
+      [
+        '#theme' => [
+          'theme_test_base5',
+          'theme_test_base4',
+          'theme_test_base3__from_theme_property',
+          'theme_test_base3',
+          'theme_test_base2__from_theme_property__without_base',
+          'theme_test_base1__from_theme_property__too',
+        ],
+      ],
+      $modules,
+      $theme,
+      $expected,
+      $unexpected
+    );
+  }
+
+  /**
+   * Data provider for testArrayThemeSuggestions().
+   *
+   * @see testArrayThemeSuggestions()
+   */
+  public function providerArrayThemeSuggestions(): array {
+    $extension = '.html.twig';
+    return [
+      'Only the last #theme array entry is expanded into suggestions' => [
+        'modules' => [],
+        'theme' => '',
+        'expected' => [
+          'Template for testing suggestion hooks when #theme contains a list of theme suggestions.',
+          '<!-- FILE NAME SUGGESTIONS:' . PHP_EOL
+          . '   * theme-test-base5' . $extension . PHP_EOL
+          . '   * theme-test-base4' . $extension . PHP_EOL
+          . '   * theme-test-base3--from-theme-property' . $extension . PHP_EOL
+          . '   * theme-test-base3' . $extension . PHP_EOL
+          . '   * theme-test-base2--from-theme-property--without-base' . $extension . PHP_EOL
+          . '   * theme-test-base1--from-theme-property--too' . $extension . PHP_EOL
+          . '   * theme-test-base1--from-theme-property' . $extension . PHP_EOL
+          . '   x theme-test-base1' . $extension . PHP_EOL
+          . '-->' . PHP_EOL,
+        ],
+        'unexpected' => [
+          'This theme_test_base2 template is implemented, but never used.',
+          'x theme-test-base2' . $extension . PHP_EOL,
+          '* theme-test-base2' . $extension . PHP_EOL,
+        ],
+      ],
+      'Confirm unexpanded theme_test_base2 suggestion would be used if expanded' => [
+        'modules' => ['theme_suggestions_base2_ignored_test'],
+        'theme' => '',
+        'expected' => [
+          'This theme_test_base2 template is implemented, but never used.',
+          '<!-- FILE NAME SUGGESTIONS:' . PHP_EOL
+          . '   * theme-test-base5' . $extension . PHP_EOL
+          . '   * theme-test-base4' . $extension . PHP_EOL
+          . '   * theme-test-base3--from-theme-property' . $extension . PHP_EOL
+          . '   * theme-test-base3' . $extension . PHP_EOL
+          . '   * theme-test-base2--from-theme-property--without-base' . $extension . PHP_EOL
+          . '   x theme-test-base2' . $extension . PHP_EOL
+          . '   * theme-test-base1--from-theme-property--too' . $extension . PHP_EOL
+          . '   * theme-test-base1--from-theme-property' . $extension . PHP_EOL
+          . '   * theme-test-base1' . $extension . PHP_EOL
+          . '-->' . PHP_EOL,
+        ],
+        'unexpected' => [],
+      ],
+      // This is a duplicate of testThemeSuggestionsOrdering, but in an array
+      // context.
+      '#theme property suggestions always override ones from hook_theme_suggestions_hook' => [
+        'modules' => ['theme_suggestions_base1_test'],
+        'theme' => '',
+        'expected' => [
+          'Template for testing suggestion hooks when #theme contains a list of theme suggestions.',
+          '<!-- FILE NAME SUGGESTIONS:' . PHP_EOL
+          . '   * theme-test-base5' . $extension . PHP_EOL
+          . '   * theme-test-base4' . $extension . PHP_EOL
+          . '   * theme-test-base3--from-theme-property' . $extension . PHP_EOL
+          . '   * theme-test-base3' . $extension . PHP_EOL
+          . '   * theme-test-base2--from-theme-property--without-base' . $extension . PHP_EOL
+          . '   * theme-test-base1--from-hook-theme-suggestions-hook-alter' . $extension . PHP_EOL
+          . '   * theme-test-base1--from-hook-theme-suggestions-hook-too' . $extension . PHP_EOL
+          . '   * theme-test-base1--from-hook-theme-suggestions-hook-alter--but-reordered' . $extension . PHP_EOL
+          . '   * theme-test-base1--from-theme-property--too' . $extension . PHP_EOL
+          . '   * theme-test-base1--from-theme-property' . $extension . PHP_EOL
+          . '   * theme-test-base1--from-hook-theme-suggestions-hook' . $extension . PHP_EOL
+          . '   x theme-test-base1' . $extension . PHP_EOL
+          . '-->' . PHP_EOL,
+        ],
+        'unexpected' => [],
+      ],
+      'specific #theme property suggestions always override ones from hook_theme_suggestions_hook' => [
+        'modules' => ['theme_suggestions_base2_test'],
+        'theme' => '',
+        'expected' => [
+          'This theme_test_base2__from_theme_property__without_base template is implemented.',
+          '<!-- FILE NAME SUGGESTIONS:' . PHP_EOL
+          . '   * theme-test-base5' . $extension . PHP_EOL
+          . '   * theme-test-base4' . $extension . PHP_EOL
+          . '   * theme-test-base3--from-theme-property' . $extension . PHP_EOL
+          . '   * theme-test-base3' . $extension . PHP_EOL
+          . '   * theme-test-base2--from-hook-theme-suggestions-hook-alter' . $extension . PHP_EOL
+          . '   x theme-test-base2--from-theme-property--without-base' . $extension . PHP_EOL
+          . '   * theme-test-base2--from-hook-theme-suggestions-hook' . $extension . PHP_EOL
+          . '   * theme-test-base1--from-theme-property--too' . $extension . PHP_EOL
+          . '   * theme-test-base1--from-theme-property' . $extension . PHP_EOL
+          . '   * theme-test-base1' . $extension . PHP_EOL
+          . '-->' . PHP_EOL,
+        ],
+        'unexpected' => [
+          'This theme_test_base2__from_hook_theme_suggestions_hook template is implemented, but never used.',
+          'This theme_test_base2 template is implemented, but never used.',
+          'x theme-test-base2' . $extension . PHP_EOL,
+          '* theme-test-base2' . $extension . PHP_EOL,
+        ],
+      ],
+      'adding a new template implementation does not change order of suggestions' => [
+        'modules' => ['theme_suggestions_base1_test'],
+        'theme' => 'test_theme',
+        'expected' => [
+          'This theme_test_base1__from_theme_property__too template is implemented.',
+          '<!-- FILE NAME SUGGESTIONS:' . PHP_EOL
+          . '   * theme-test-base5' . $extension . PHP_EOL
+          . '   * theme-test-base4' . $extension . PHP_EOL
+          . '   * theme-test-base3--from-theme-property' . $extension . PHP_EOL
+          . '   * theme-test-base3' . $extension . PHP_EOL
+          . '   * theme-test-base2--from-theme-property--without-base' . $extension . PHP_EOL
+          . '   * theme-test-base1--from-hook-theme-suggestions-hook-alter' . $extension . PHP_EOL
+          . '   * theme-test-base1--from-hook-theme-suggestions-hook-too' . $extension . PHP_EOL
+          . '   * theme-test-base1--from-hook-theme-suggestions-hook-alter--but-reordered' . $extension . PHP_EOL
+          . '   x theme-test-base1--from-theme-property--too' . $extension . PHP_EOL
+          . '   * theme-test-base1--from-theme-property' . $extension . PHP_EOL
+          . '   * theme-test-base1--from-hook-theme-suggestions-hook' . $extension . PHP_EOL
+          . '   * theme-test-base1' . $extension . PHP_EOL
+          . '-->' . PHP_EOL,
+        ],
+        'unexpected' => [],
+      ],
+      'theme suggestions alter hooks see suggestions from #theme' => [
+        'modules' => ['theme_suggestions_base3_test'],
+        'theme' => '',
+        'expected' => [
+          'This theme_test_base3 template is implemented.',
+          '<!-- FILE NAME SUGGESTIONS:' . PHP_EOL
+          . '   * theme-test-base5' . $extension . PHP_EOL
+          . '   * theme-test-base4' . $extension . PHP_EOL
+          . '   * theme-test-base3--from-hook-theme-suggestions-hook-alter' . $extension . PHP_EOL
+          . '   * theme-test-base3--from-theme-property--seen-by-alter' . $extension . PHP_EOL
+          . '   x theme-test-base3' . $extension . PHP_EOL,
+        ],
+        'unexpected' => [],
+      ],
+      'different "base hook" specified in theme registry' => [
+        'modules' => ['theme_suggestions_base4_test'],
+        'theme' => '',
+        'expected' => [
+          'This theme_test_base4 template is implemented and has a different "base hook".',
+          '<!-- FILE NAME SUGGESTIONS:' . PHP_EOL
+          . '   * theme-test-base5' . $extension . PHP_EOL
+          . '   * theme-suggestions-base4-test-alternate--from-hook-theme-suggestions-hook-alter' . $extension . PHP_EOL
+          . '   x theme-test-base4' . $extension . PHP_EOL
+          . '   * theme-suggestions-base4-test-alternate--from-hook-theme-suggestions-hook' . $extension . PHP_EOL
+          // 'theme-suggestions-base4-test-alternate' is not listed here. While
+          // it would seem to make sense to list the base hook here, when one
+          // hook specifies a different base hook, Drupal will use that base
+          // hook's suggestions, but not the base hook itself.
+          . '   * theme-test-base3--from-theme-property' . $extension . PHP_EOL,
+        ],
+        'unexpected' => [],
+      ],
+      'implementation of base hook template not used when different "base hook" specified in theme registry' => [
+        'modules' => ['theme_suggestions_base4_test'],
+        'theme' => 'test_theme',
+        'expected' => [
+          'This theme_test_base4 template is implemented and has a different "base hook".',
+          '<!-- FILE NAME SUGGESTIONS:' . PHP_EOL
+          . '   * theme-test-base5' . $extension . PHP_EOL
+          . '   * theme-suggestions-base4-test-alternate--from-hook-theme-suggestions-hook-alter' . $extension . PHP_EOL
+          . '   x theme-test-base4' . $extension . PHP_EOL
+          . '   * theme-suggestions-base4-test-alternate--from-hook-theme-suggestions-hook' . $extension . PHP_EOL
+          // 'theme-suggestions-base4-test-alternate' is not listed here. While
+          // it would seem to make sense to list the base hook here, when one
+          // hook specifies a different base hook, Drupal will use that base
+          // hook's suggestions, but not the base hook itself.
+          . '   * theme-test-base3--from-theme-property' . $extension . PHP_EOL,
+        ],
+        'unexpected' => [
+          'This theme_suggestions_base4_test_alternate template is implemented, but not used.',
+        ],
+      ],
+      'implementation of base hook suggestion used when different "base hook" specified in theme registry' => [
+        'modules' => [
+          'theme_suggestions_base1_test',
+          'theme_suggestions_base4_test',
+        ],
+        'theme' => 'test_theme',
+        'expected' => [
+          'This theme_suggestions_base4_test_alternate__from_hook_theme_suggestions_hook_alter template is implemented.',
+          '<!-- FILE NAME SUGGESTIONS:' . PHP_EOL
+          . '   * theme-test-base5' . $extension . PHP_EOL
+          . '   x theme-suggestions-base4-test-alternate--from-hook-theme-suggestions-hook-alter' . $extension . PHP_EOL
+          . '   * theme-test-base4' . $extension . PHP_EOL
+          . '   * theme-suggestions-base4-test-alternate--from-hook-theme-suggestions-hook' . $extension . PHP_EOL
+          // 'theme-suggestions-base4-test-alternate' is not listed here. While
+          // it would seem to make sense to list the base hook here, when one
+          // hook specifies a different base hook, Drupal will use that base
+          // hook's suggestions, but not the base hook itself.
+          . '   * theme-test-base3--from-theme-property' . $extension . PHP_EOL,
+        ],
+        'unexpected' => [],
+      ],
+    ];
+  }
+
+  /**
+   * Tests execution order of theme suggestion hooks.
+   *
+   * @throws \Exception
+   */
+  public function testExecutionOrder() {
+    // Normal module weight is not calculated in KernelTest, so we fake it by
+    // (re)installing the modules in order of their weight (alphabetical order).
+    $this->disableModules(['theme_test']);
+    $this->enableModules(['theme_suggestions_test', 'theme_test']);
+    $theme = 'test_theme';
+    $this->container->get('theme_installer')->install([$theme]);
+    $this->config('system.theme')->set('default', $theme)->save();
+
+    $build = [
+      '#theme' => 'theme_test_suggestions',
+    ];
+
+    // Messages are not output with $this->render().
+    $output = $this->render($build);
+
+    $this->assertStringContainsString('Template overridden based on new theme suggestion provided by the test_theme theme via hook_theme_suggestions_HOOK_alter().', $output, $this->getName());
+
+    // Retrieve all messages we've set via \Drupal::messenger()->addStatus().
+    $messages = $this->container->get('messenger')->messagesByType('status');
+
+    // Ensure that the order is:
+    // 1. hook_theme_suggestions_HOOK()
+    // 2. Grouped by module:
+    //    hook_theme_suggestions_alter()
+    //    hook_theme_suggestions_HOOK_alter()
+    // 3. Grouped by theme:
+    //    hook_theme_suggestions_alter()
+    //    hook_theme_suggestions_HOOK_alter()
+    $expected_order = [
+      'theme_test_theme_suggestions_theme_test_suggestions() executed.',
+      'theme_suggestions_test_theme_suggestions_alter() executed.',
+      'theme_suggestions_test_theme_suggestions_theme_test_suggestions_alter() executed.',
+      'theme_test_theme_suggestions_alter() executed.',
+      'theme_test_theme_suggestions_theme_test_suggestions_alter() executed.',
+      'test_theme_theme_suggestions_alter() executed.',
+      'test_theme_theme_suggestions_theme_test_suggestions_alter() executed.',
+    ];
+    $this->assertEquals($expected_order, $messages);
+  }
+
+}
diff --git a/core/modules/system/tests/src/Kernel/Theme/TwigDebugMarkupTest.php b/core/modules/system/tests/src/Kernel/Theme/TwigDebugMarkupTest.php
new file mode 100644
index 0000000000..a0732cced4
--- /dev/null
+++ b/core/modules/system/tests/src/Kernel/Theme/TwigDebugMarkupTest.php
@@ -0,0 +1,167 @@
+<?php
+
+namespace Drupal\Tests\system\Kernel\Theme;
+
+use Drupal\Component\Utility\Html;
+use Drupal\Core\DependencyInjection\ContainerBuilder;
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * Tests for Twig debug markup.
+ *
+ * @group Theme
+ */
+class TwigDebugMarkupTest extends KernelTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  protected static $modules = ['system', 'theme_test'];
+
+  /**
+   * {@inheritdoc}
+   */
+  public function register(ContainerBuilder $container) {
+    parent::register($container);
+    // Enable Twig debugging.
+    $parameters = $container->getParameter('twig.config');
+    $parameters['debug'] = TRUE;
+    $container->setParameter('twig.config', $parameters);
+  }
+
+  /**
+   * {@inheritdoc}
+   *
+   * We override KernelTestBase::render() so that it outputs Twig debug comments
+   * only for the render array given in a test and not for an entire page.
+   */
+  protected function render(array &$elements): string {
+    return $this->container->get('renderer')->renderRoot($elements);
+  }
+
+  /**
+   * Tests debug markup is on.
+   *
+   * @throws \Exception
+   */
+  public function testDebugMarkup() {
+    $extension = '.html.twig';
+    $hook = 'theme_test_specific_suggestions';
+    $build = [
+      '#theme' => $hook,
+    ];
+
+    // Find full path to template.
+    $cache = $this->container->get('theme.registry')->get();
+    $templates = drupal_find_theme_templates($cache, $extension, $this->container->get('extension.list.module')->getPath('theme_test'));
+    $template_filename = $templates[$hook]['path'] . '/' . $templates[$hook]['template'] . $extension;
+
+    // Render a template.
+    $output = $this->render($build);
+
+    $expected = '<!-- THEME DEBUG -->';
+    $this->assertStringContainsString($expected, $output, 'Twig debug markup found in theme output when debug is enabled.');
+
+    $expected = "\n<!-- THEME HOOK: '$hook' -->";
+    $this->assertStringContainsString($expected, $output, 'Theme hook comment found.');
+
+    $expected = "\n<!-- BEGIN OUTPUT from '" . Html::escape($template_filename) . "' -->\n";
+    $this->assertStringContainsString($expected, $output, 'Full path to current template file found in BEGIN OUTPUT comment.');
+    $expected = "\n<!-- END OUTPUT from '" . Html::escape($template_filename) . "' -->\n";
+    $this->assertStringContainsString($expected, $output, 'Full path to current template file found in END OUTPUT comment.');
+  }
+
+  /**
+   * Tests file name suggestions comment.
+   *
+   * @throws \Exception
+   */
+  public function testFileNameSuggestions() {
+    $extension = '.html.twig';
+
+    // Render a template using a single suggestion.
+    $build = [
+      '#theme' => 'theme_test_specific_suggestions',
+    ];
+    $output = $this->render($build);
+
+    $expected = "\n<!-- THEME HOOK: 'theme_test_specific_suggestions' -->";
+    $this->assertStringContainsString($expected, $output, 'Theme hook comment found.');
+    $unexpected = '<!-- FILE NAME SUGGESTIONS:';
+    $this->assertStringNotContainsString($unexpected, $output, 'A single suggestion should not have file name suggestions listed.');
+
+    // Render a template using multiple suggestions.
+    $build = [
+      '#theme' => 'theme_test_specific_suggestions__variant',
+    ];
+    $output = $this->render($build);
+
+    $expected = "\n<!-- THEME HOOK: 'theme_test_specific_suggestions__variant' -->";
+    $this->assertStringContainsString($expected, $output, 'Theme hook comment found.');
+    $expected = '<!-- FILE NAME SUGGESTIONS:' . PHP_EOL
+      . '   * theme-test-specific-suggestions--variant' . $extension . PHP_EOL
+      . '   x theme-test-specific-suggestions' . $extension . PHP_EOL
+      . '-->';
+    $this->assertStringContainsString($expected, $output, 'Multiple suggestions should have file name suggestions listed.');
+  }
+
+  /**
+   * Tests suggestions when file name does not match.
+   *
+   * @throws \Exception
+   */
+  public function testFileNameNotMatchingSuggestion() {
+    $extension = '.html.twig';
+
+    // Find full path to template.
+    $cache = $this->container->get('theme.registry')->get();
+    $templates = drupal_find_theme_templates($cache, $extension, $this->container->get('extension.list.module')->getPath('theme_test'));
+    $template_filename = $templates['theme_test_template_test']['path'] . '/' . $templates['theme_test_template_test']['template'] . $extension;
+
+    // Render a template that doesn't match its suggestion name.
+    $build = [
+      '#theme' => 'theme_test_template_test__variant',
+    ];
+    $output = $this->render($build);
+
+    $expected = "\n<!-- THEME HOOK: 'theme_test_template_test__variant' -->";
+    $this->assertStringContainsString($expected, $output, 'Theme hook comment found.');
+
+    $expected = '<!-- FILE NAME SUGGESTIONS:' . PHP_EOL
+      . '   * theme-test-template-test--variant' . $extension . PHP_EOL
+      . '   x theme_test.template_test' . $extension . PHP_EOL
+      . '-->';
+    $this->assertStringContainsString($expected, $output, 'The actual template file name should be used when it does not match the suggestion.');
+
+    $expected = "\n<!-- BEGIN OUTPUT from '" . Html::escape($template_filename) . "' -->\n";
+    $this->assertStringContainsString($expected, $output, 'Full path to current template file found in BEGIN OUTPUT comment.');
+  }
+
+  /**
+   * Tests XSS attempt in theme suggestions and Twig debug comments.
+   *
+   * @throws \Exception
+   */
+  public function testXssComments() {
+    $extension = '.html.twig';
+
+    // Render a template whose suggestions have been compromised.
+    $build = [
+      '#theme' => 'theme_test_xss_suggestion',
+    ];
+    $output = $this->render($build);
+
+    // @see theme_test_theme_suggestions_node()
+    $xss_suggestion = Html::escape('theme-test-xss-suggestion--<script type="text/javascript">alert(\'yo\');</script>') . $extension;
+
+    $expected = '<!-- FILE NAME SUGGESTIONS:' . PHP_EOL
+      . '   * ' . $xss_suggestion . PHP_EOL
+      . '   x theme-test-xss-suggestion' . $extension . PHP_EOL
+      . '-->';
+    $this->assertStringContainsString($expected, $output, 'XSS suggestion successfully escaped in Twig debug comments.');
+    $this->assertStringContainsString('Template for testing XSS in theme hook suggestions.', $output, 'Base hook suggestion used instead of XSS suggestion.');
+  }
+
+}
diff --git a/core/modules/system/tests/themes/test_theme/templates/theme-suggestions-base4-test-alternate.html.twig b/core/modules/system/tests/themes/test_theme/templates/theme-suggestions-base4-test-alternate.html.twig
new file mode 100644
index 0000000000..e8d83a2774
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme/templates/theme-suggestions-base4-test-alternate.html.twig
@@ -0,0 +1,2 @@
+{# Output for Theme API test #}
+This theme_suggestions_base4_test_alternate template is implemented, but not used.
diff --git a/core/modules/system/tests/themes/test_theme/templates/theme-test-base1--from-theme-property--too.html.twig b/core/modules/system/tests/themes/test_theme/templates/theme-test-base1--from-theme-property--too.html.twig
new file mode 100644
index 0000000000..4b6e3745af
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme/templates/theme-test-base1--from-theme-property--too.html.twig
@@ -0,0 +1,2 @@
+{# Output for Theme API test #}
+This theme_test_base1__from_theme_property__too template is implemented.
diff --git a/core/modules/system/tests/themes/test_theme/templates/theme-test-specific-suggestions--variant--foo.html.twig b/core/modules/system/tests/themes/test_theme/templates/theme-test-specific-suggestions--variant--foo.html.twig
index a464e47a51..0a0408d7ae 100644
--- a/core/modules/system/tests/themes/test_theme/templates/theme-test-specific-suggestions--variant--foo.html.twig
+++ b/core/modules/system/tests/themes/test_theme/templates/theme-test-specific-suggestions--variant--foo.html.twig
@@ -1,5 +1,2 @@
 {# Output for Theme API test #}
-Template overridden based on suggestion alter hook determined by the base hook.
-
-<p>Theme hook suggestions:
-{{ theme_hook_suggestions|safe_join("<br />") }}</p>
+Template overridden based on suggestion alter hook determined by a module's hook_theme_suggestions_HOOK_alter().
diff --git a/core/modules/system/tests/themes/test_theme/templates/theme-test-specific-suggestions--variant.html.twig b/core/modules/system/tests/themes/test_theme/templates/theme-test-specific-suggestions--variant.html.twig
index 8ac8cd241a..f0de75dff0 100644
--- a/core/modules/system/tests/themes/test_theme/templates/theme-test-specific-suggestions--variant.html.twig
+++ b/core/modules/system/tests/themes/test_theme/templates/theme-test-specific-suggestions--variant.html.twig
@@ -1,5 +1,2 @@
 {# Output for Theme API test #}
-Template matching the specific theme call.
-
-<p>Theme hook suggestions:
-{{ theme_hook_suggestions|safe_join("<br />") }}</p>
+Template overridden based on suggestion alter hook determined by the base hook.
diff --git a/core/modules/system/tests/themes/test_theme/test_theme.theme b/core/modules/system/tests/themes/test_theme/test_theme.theme
index 78359a8020..58ae0bbd21 100644
--- a/core/modules/system/tests/themes/test_theme/test_theme.theme
+++ b/core/modules/system/tests/themes/test_theme/test_theme.theme
@@ -60,13 +60,9 @@ function test_theme_theme_suggestions_alter(array &$suggestions, array &$variabl
 /**
  * Implements hook_theme_suggestions_HOOK_alter().
  */
-function test_theme_theme_suggestions_theme_test_suggestions_alter(array &$suggestions, array $variables) {
+function test_theme_theme_suggestions_theme_test_suggestions_alter(array &$suggestions, array $variables, $hook) {
   \Drupal::messenger()->addStatus(__FUNCTION__ . '() executed.');
-  // Theme alter hooks run after module alter hooks, so add this theme
-  // suggestion to the beginning of the array so that the suggestion added by
-  // the theme_suggestions_test module can be picked up when that module is
-  // enabled.
-  array_unshift($suggestions, 'theme_test_suggestions__' . 'theme_override');
+  $suggestions[] = $hook . '__theme_override';
 }
 
 /**
diff --git a/core/modules/views/tests/modules/views_test_config/test_views/views.view.test_page_display.yml b/core/modules/views/tests/modules/views_test_config/test_views/views.view.test_page_display.yml
index 858cb58940..2068f9efb3 100644
--- a/core/modules/views/tests/modules/views_test_config/test_views/views.view.test_page_display.yml
+++ b/core/modules/views/tests/modules/views_test_config/test_views/views.view.test_page_display.yml
@@ -5,7 +5,7 @@ id: test_page_display
 label: test_page_display
 module: views
 description: ''
-tag: ''
+tag: 'tag1,tag2'
 base_table: views_test_data
 base_field: nid
 display:
diff --git a/core/modules/views/tests/src/Kernel/ViewsTemplateTest.php b/core/modules/views/tests/src/Kernel/ViewsTemplateTest.php
index 37a057a7dd..d21be68412 100644
--- a/core/modules/views/tests/src/Kernel/ViewsTemplateTest.php
+++ b/core/modules/views/tests/src/Kernel/ViewsTemplateTest.php
@@ -66,4 +66,30 @@ public function testThemeSuggestionsContainerAlter() {
     $this->assertStringContainsString($expected, $output, 'Views more link container suggestions found in Twig debug output');
   }
 
+  /**
+   * Tests the views theme suggestions.
+   *
+   * @throws \Exception
+   */
+  public function testThemeSuggestionsViewsView() {
+    $build = [
+      '#type' => 'view',
+      '#name' => 'test_page_display',
+      '#display_id' => 'default',
+      '#arguments' => [],
+    ];
+
+    $output = $this->render($build);
+    $extension = '.html.twig';
+    $expected = '<!-- FILE NAME SUGGESTIONS:' . PHP_EOL
+      . '   * views-view--test-page-display--default' . $extension . PHP_EOL
+      . '   * views-view--default' . $extension . PHP_EOL
+      . '   * views-view--tag1' . $extension . PHP_EOL
+      . '   * views-view--tag2' . $extension . PHP_EOL
+      . '   * views-view--test-page-display' . $extension . PHP_EOL
+      . '   x views-view' . $extension . PHP_EOL
+      . '-->' . PHP_EOL;
+    $this->assertStringContainsString($expected, $output, 'Views theme suggestions found in Twig debug output');
+  }
+
 }
diff --git a/core/themes/engines/twig/twig.engine b/core/themes/engines/twig/twig.engine
index f0581b15cd..aab12c2a08 100644
--- a/core/themes/engines/twig/twig.engine
+++ b/core/themes/engines/twig/twig.engine
@@ -51,55 +51,21 @@ function twig_render_template($template_file, array $variables) {
   if ($twig_service->isDebug()) {
     $output['debug_prefix'] .= "\n\n<!-- THEME DEBUG -->";
     $output['debug_prefix'] .= "\n<!-- THEME HOOK: '" . Html::escape($variables['theme_hook_original']) . "' -->";
-    // If there are theme suggestions, reverse the array so more specific
-    // suggestions are shown first.
-    if (!empty($variables['theme_hook_suggestions'])) {
-      $variables['theme_hook_suggestions'] = array_reverse($variables['theme_hook_suggestions']);
-    }
-    // Add debug output for directly called suggestions like
-    // '#theme' => 'comment__node__article'.
-    if (str_contains($variables['theme_hook_original'], '__')) {
-      $derived_suggestions[] = $hook = $variables['theme_hook_original'];
-      while ($pos = strrpos($hook, '__')) {
-        $hook = substr($hook, 0, $pos);
-        $derived_suggestions[] = $hook;
-      }
-      // Get the value of the base hook (last derived suggestion) and append it
-      // to the end of all theme suggestions.
-      $base_hook = array_pop($derived_suggestions);
-      $variables['theme_hook_suggestions'] = array_merge($derived_suggestions, $variables['theme_hook_suggestions']);
-      $variables['theme_hook_suggestions'][] = $base_hook;
-    }
-    if (!empty($variables['theme_hook_suggestions'])) {
+    // If there is only one template suggestion file, we don't output it, since
+    // it would be somewhat redundant with the THEME HOOK above.
+    if (count($variables['template_suggestions']) > 1) {
       $extension = twig_extension();
       $current_template = basename($template_file);
-      $suggestions = $variables['theme_hook_suggestions'];
-      // Only add the original theme hook if it wasn't a directly called
-      // suggestion.
-      if (!str_contains($variables['theme_hook_original'], '__')) {
-        $suggestions[] = $variables['theme_hook_original'];
-      }
-      $invalid_suggestions = [];
-      $base_hook = $base_hook ?? $variables['theme_hook_original'];
-      foreach ($suggestions as $key => &$suggestion) {
-        // Valid suggestions are $base_hook, $base_hook__*, and contain no hyphens.
-        if (($suggestion !== $base_hook && !str_starts_with($suggestion, $base_hook . '__')) || str_contains($suggestion, '-')) {
-          $invalid_suggestions[] = $suggestion;
-          unset($suggestions[$key]);
-          continue;
+      $suggestions = $variables['template_suggestions'];
+      foreach ($suggestions as &$suggestion) {
+        if ($suggestion === $variables['template_suggestion']) {
+          $suggestion = 'x ' . $current_template;
+        }
+        else {
+          $suggestion = '* ' . strtr($suggestion, '_', '-') . $extension;
         }
-        $template = strtr($suggestion, '_', '-') . $extension;
-        $prefix = ($template == $current_template) ? 'x' : '*';
-        $suggestion = $prefix . ' ' . $template;
       }
       $output['debug_info'] .= "\n<!-- FILE NAME SUGGESTIONS:\n   " . Html::escape(implode("\n   ", $suggestions)) . "\n-->";
-
-      if (!empty($invalid_suggestions)) {
-        $output['debug_info'] .= "\n<!-- INVALID FILE NAME SUGGESTIONS:";
-        $output['debug_info'] .= "\n   See https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Render!theme.api.php/function/hook_theme_suggestions_alter";
-        $output['debug_info'] .= "\n   " . Html::escape(implode("\n   ", $invalid_suggestions));
-        $output['debug_info'] .= "\n-->";
-      }
     }
     $output['debug_info']   .= "\n<!-- BEGIN OUTPUT from '" . Html::escape($template_file) . "' -->\n";
     $output['debug_suffix'] .= "\n<!-- END OUTPUT from '" . Html::escape($template_file) . "' -->\n\n";