Issue #2614014 by joelpittet, alexpott, dawehner, Cottser, LewisNyman: Progress bar, fieldsets, messages broken in the installer due to theme ordering bug

core/includes/theme.maintenance.inc

@@ -82,19 +82,21 @@ function _drupal_maintenance_theme() {
   $theme = $custom_theme;
 
   // Find all our ancestor themes and put them in an array.
-  $base_theme = array();
+  // @todo This is just a workaround. Find a better way how to handle themes
+  //   on maintenance pages, see https://www.drupal.org/node/2322619.
+  // This code is basically a duplicate of
+  // \Drupal\Core\Theme\ThemeInitialization::getActiveThemeByName.
+  $base_themes = [];
   $ancestor = $theme;
   while ($ancestor && isset($themes[$ancestor]->base_theme)) {
-    $base_theme[] = $themes[$themes[$ancestor]->base_theme];
+    $base_themes[] = $themes[$themes[$ancestor]->base_theme];
     $ancestor = $themes[$ancestor]->base_theme;
     if ($ancestor) {
-      // Ensure that the base theme is added.
+      // Ensure that the base theme is added and installed.
       $theme_handler->addTheme($themes[$ancestor]);
     }
   }
-  // @todo This is just a workaround. Find a better way how to handle themes
-  //   on maintenance pages, see https://www.drupal.org/node/2322619.
-  \Drupal::theme()->setActiveTheme($theme_init->getActiveTheme($themes[$custom_theme], array_reverse($base_theme)));
+  \Drupal::theme()->setActiveTheme($theme_init->getActiveTheme($themes[$custom_theme], $base_themes));
   // Prime the theme registry.
   Drupal::service('theme.registry');
 }

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

@@ -57,8 +57,8 @@ interface ThemeInitializationInterface {
    * @param \Drupal\Core\Extension\Extension $theme
    *   The theme extension object.
    * @param \Drupal\Core\Extension\Extension[] $base_themes
-   *    An array of extension objects of base theme and its bases. It is ordered
-   *    by 'oldest first', meaning the top level of the chain will be first.
+   *   An array of extension objects of base theme and its bases. It is ordered
+   *   by 'next parent first', meaning the top level of the chain will be first.
    *
    * @return \Drupal\Core\Theme\ActiveTheme
    *   The active theme instance for the passed in $theme.

core/modules/system/src/Tests/Installer/StandardInstallerTest.php

@@ -40,6 +40,19 @@ class StandardInstallerTest extends ConfigAfterInstallerTestBase {
     parent::setUpSite();
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  protected function curlExec($curl_options, $redirect = FALSE) {
+    // Ensure that we see the classy progress CSS on the batch page.
+    // Batch processing happens as part of HTTP redirects, so we can access the
+    // HTML of the batch page.
+    if (strpos($curl_options[CURLOPT_URL], '&id=1&op=do_nojs') !== FALSE) {
+      $this->assertRaw('themes/classy/css/components/progress.css');
+    }
+    return parent::curlExec($curl_options, $redirect);
+  }
+
   /**
    * Ensures that the exported standard configuration is up to date.
    */

core/tests/Drupal/KernelTests/Core/Theme/MaintenanceThemeTest.php

@@ -0,0 +1,43 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\KernelTests\Core\Theme\MaintenanceThemeTest.
+ */
+
+namespace Drupal\KernelTests\Core\Theme;
+
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * Tests themes and base themes are correctly loaded.
+ *
+ * @group Installer
+ */
+class MaintenanceThemeTest extends KernelTestBase {
+
+  /**
+   * Tests that the maintenance theme initializes the theme and its base themes.
+   */
+  public function testMaintenanceTheme() {
+    $this->setSetting('maintenance_theme', 'seven');
+    // Get the maintenance theme loaded.
+    drupal_maintenance_theme();
+
+    // Do we have an active theme?
+    $this->assertTrue(\Drupal::theme()->hasActiveTheme());
+
+    $active_theme = \Drupal::theme()->getActiveTheme();
+    $this->assertEquals('seven', $active_theme->getName());
+
+    $base_themes = $active_theme->getBaseThemes();
+    $base_theme_names = array_keys($base_themes);
+    $this->assertSame(['classy', 'stable'], $base_theme_names);
+
+    // Ensure Classy has the correct base themes and amount of base themes.
+    $classy_base_themes = $base_themes['classy']->getBaseThemes();
+    $classy_base_theme_names = array_keys($classy_base_themes);
+    $this->assertSame(['stable'], $classy_base_theme_names);
+  }
+
+}