YAML discovery does not take theme inheritance into account

Provided a MR with a failing test to demonstrate the problem.

https://git.drupalcode.org/issue/drupal-3457863/-/jobs/1984798#L4263

---- Drupal\KernelTests\Core\Layout\LayoutPluginManagerTest ----
Status    Group      Filename          Line Function                            
--------------------------------------------------------------------------------
Fail      Other      phpunit-361.xml      0 Drupal\KernelTests\Core\Layout\Layo
    PHPUnit Test failed to complete; Error: PHPUnit 10.5.20 by Sebastian
    Bergmann and contributors.
    
    Runtime:       PHP 8.3.8
    Configuration: /builds/issue/drupal-3457863/core/phpunit.xml.dist
    
    F                                                                   1 / 1
    (100%)
    
    Time: 00:01.315, Memory: 8.00 MB
    
    There was 1 failure:
    
    1)
    Drupal\KernelTests\Core\Layout\LayoutPluginManagerTest::testPluginOverride
    Failed asserting that two strings are identical.
    --- Expected
    +++ Actual
    @@ @@
    -'Child'
    +'Parent'
    
    /builds/issue/drupal-3457863/core/tests/Drupal/KernelTests/Core/Layout/LayoutPluginManagerTest.php:40

Copy pasting works from @maboy in #3263526: UX: Add other widgets than select, MR 47.

/**
   * Get list of theme directories sorted by inheritance.
   *
   * @return array
   *   The sorted directories.
   */
  protected function orderTheme(): array {
    // Order theme directories to allow overriding in subtheme.
    $themes = $this->themeHandler->listInfo();
    $ordered_theme_directories = [];
    $base_theme = [];
    $theme_directories = $this->themeHandler->getThemeDirectories();

    // Create a list which includes the current theme and all its base themes.
    foreach ($themes as $key => $theme) {
      // Base theme for current theme.
      // We need to place current theme AFTER the base theme in array.
      if (isset($theme->base_themes)) {
        // Get the last base_theme.
        // To get the key after we want to insert the current theme.
        $after_key = \array_key_last($theme->base_themes);
        $index = \array_search($after_key, \array_keys($ordered_theme_directories), TRUE);

        // The current theme is a base theme.
        $before_key = NULL;
        if (\array_key_exists($key, $base_theme)) {
          $before_key = $base_theme[$key];
        }

        // The base theme is not existing yet in the array.
        if ($index == 0) {
          // Current theme is also a base theme.
          if ($before_key) {
            // Insert the current theme before its children in the array.
            $index_before = \array_search($before_key, \array_keys($ordered_theme_directories), TRUE);
            $ordered_theme_directories = \array_slice($ordered_theme_directories, 0, $index_before - 1) + [$key => $theme_directories[$key]] + $ordered_theme_directories;
          }
          else {
            // Insert the current theme at the end of the array.
            $ordered_theme_directories[$key] = $theme_directories[$key];
          }
          $base_theme = [$after_key => [$key]];
        }
        // The base theme exist in the array.
        else {
          // Insert the current theme after the base_theme.
          $ordered_theme_directories = \array_slice($ordered_theme_directories, 0, $index + 1) + [$key => $theme_directories[$key]] + $ordered_theme_directories;
        }
      }
      // No base theme,
      // Can be added first.
      else {
        $ordered_theme_directories = [$key => $theme_directories[$key]] + $ordered_theme_directories;
      }
    }

    return $ordered_theme_directories;
  }

I will try to use that for a generic fix.

I have just put code from comment 6 into the getThemeDirectories method of the themeHandler directly.

I have changed the logic to not rely on itself anymore to avoid infinite loop. And so I had to change some variables name. I hope the logic is still ok.

At the beginning I tried a "simplified" approach based on:

    $theme_config = $this->configFactory->get('system.theme');
    $default_theme = $theme_config->get('default');
    $admin_theme = $theme_config->get('admin');

And only handle those themes but the problem is that it can be null, at least in the tests.

Test are green now. Thanks @maboy!

So, testing with UI Suite modules, I started with UI Styles.

No problem on the default theme which is a sub theme of ui_suite_bootstrap generated using its starterkit.

But if I go on a page which should be contextualized on ui_suite_bootstrap, it does not detect the styles overridden by its sub theme.

See attached screenshots. Also I attached the styles of the sub theme.

So maybe the envisaged solution is not ok and all plugins provided by YAML should follow SDC logic to have the provider automatically prefixed in the plugin machine name to avoid overlaps.

Review comments addressed.

Thanks for the RTBC but I don't know if it's not too soon. See comment 12.

At least in RTBC this will more attract attention from maintainers to get feedbacks.

Adding the parent theme in the dependencies list of the child theme has no effect.

The pipeline failed on one test: Drupal\Tests\ckeditor5\FunctionalJavascript\ImageUrlProvider, I don't think it is related to the change in this issue.

Existing kernel test updated.

Pipeline still gets some random failure from other unrelated tests.

Suggestion approved, thanks!

Thanks a lot for the merge and the backports!