Issue #3073053 by pivica, Primsi, unstatu, pooja saraah, smustgrave, Berdir: Theme library override can fail in when you have multiple parent themes

core/lib/Drupal/Core/Theme/ActiveTheme.php

@@ -183,7 +183,8 @@ class ActiveTheme {
    * Returns an array of base theme extension objects keyed by name.
    *
    * The order starts with the base theme of $this and ends with the root of
-   * the dependency chain.
+   * the dependency chain. For most use cases, parent themes are expected to
+   * be called first, so this order needs to be reversed with array_reverse()
    *
    * @return \Drupal\Core\Extension\Extension[]
    */

core/lib/Drupal/Core/Theme/ThemeInitialization.php

@@ -131,14 +131,14 @@ class ThemeInitialization implements ThemeInitializationInterface {
     if ($active_theme->getEngine()) {
       // Include the engine.
       include_once $this->root . '/' . $active_theme->getOwner();
-      foreach ($active_theme->getBaseThemeExtensions() as $base) {
+      foreach (array_reverse($active_theme->getBaseThemeExtensions()) as $base) {
         $base->load();
       }
       $active_theme->getExtension()->load();
     }
     else {
       // include non-engine theme files
-      foreach ($active_theme->getBaseThemeExtensions() as $base) {
+      foreach (array_reverse($active_theme->getBaseThemeExtensions()) as $base) {
         // Include the theme file or the engine.
         if ($base->owner) {
           include_once $this->root . '/' . $base->owner;
@@ -178,7 +178,7 @@ class ThemeInitialization implements ThemeInitializationInterface {
     $values['libraries_override'] = [];
 
     // Get libraries overrides declared by base themes.
-    foreach ($base_themes as $base) {
+    foreach (array_reverse($base_themes) as $base) {
       if (!empty($base->info['libraries-override'])) {
         foreach ($base->info['libraries-override'] as $library => $override) {
           $values['libraries_override'][$base->getPath()][$library] = $override;

core/lib/Drupal/Core/Theme/ThemeManager.php

@@ -436,7 +436,7 @@ class ThemeManager implements ThemeManagerInterface {
       }
     }
 
-    $theme_keys = array_keys($theme->getBaseThemeExtensions());
+    $theme_keys = array_reverse(array_keys($theme->getBaseThemeExtensions()));
     $theme_keys[] = $theme->getName();
     $functions = [];
     foreach ($theme_keys as $theme_key) {

core/modules/system/tests/themes/test_subsubtheme/test_subsubtheme.info.yml

@@ -3,3 +3,8 @@ type: theme
 description: 'Test theme which uses test_subtheme as the base theme.'
 version: VERSION
 base theme: test_subtheme
+libraries-override:
+  test_basetheme/global-styling:
+    css:
+      base:
+        css/sub-sub-last-override.css: false

core/modules/views/src/ViewExecutable.php

@@ -1485,7 +1485,7 @@ class ViewExecutable {
     // @todo In the long run, it would be great to execute a view without
     //   the theme system at all. See https://www.drupal.org/node/2322623.
     $active_theme = \Drupal::theme()->getActiveTheme();
-    $themes = array_keys($active_theme->getBaseThemeExtensions());
+    $themes = array_reverse(array_keys($active_theme->getBaseThemeExtensions()));
     $themes[] = $active_theme->getName();
 
     // Check for already-cached output.

core/tests/Drupal/KernelTests/Core/Asset/LibraryDiscoveryIntegrationTest.php

@@ -127,6 +127,29 @@ class LibraryDiscoveryIntegrationTest extends KernelTestBase {
     }
   }
 
+  /**
+   * Tests libraries overrides with multiple parent themes.
+   */
+  public function testLibrariesOverridesMultiple() {
+    /** @var \Drupal\Core\Extension\ThemeInstallerInterface $theme_installer */
+    $theme_installer = $this->container->get('theme_installer');
+    $theme_installer->install(['test_basetheme']);
+    $theme_installer->install(['test_subtheme']);
+    $theme_installer->install(['test_subsubtheme']);
+
+    /** @var \Drupal\Core\Theme\ThemeInitializationInterface $theme_initializer */
+    $theme_initializer = $this->container->get('theme.initialization');
+    $active_theme = $theme_initializer->initTheme('test_subsubtheme');
+
+    $libraries_override = $active_theme->getLibrariesOverride();
+    $expected_order = [
+      'core/modules/system/tests/themes/test_basetheme',
+      'core/modules/system/tests/themes/test_subtheme',
+      'core/modules/system/tests/themes/test_subsubtheme',
+    ];
+    $this->assertEquals($expected_order, array_keys($libraries_override));
+  }
+
   /**
    * Tests library assets with other ways for specifying paths.
    */