class BlockAddTest

class BlockAddTest extends WebDriverTestBase {
    
    /**
     * {@inheritdoc}
     */
    protected static $modules = [
        'block',
        'user',
    ];
    
    /**
     * {@inheritdoc}
     */
    protected $defaultTheme = 'stark';
    
    /**
     * Tests the AJAX for the theme selector.
     */
    public function testBlockAddThemeSelector() : void {
        \Drupal::service('theme_installer')->install([
            'claro',
        ]);
        $this->drupalLogin($this->drupalCreateUser([
            'administer blocks',
        ]));
        $this->drupalGet('admin/structure/block/add/system_powered_by_block');
        $assert_session = $this->assertSession();
        // Pick a theme with a region that does not exist in another theme.
        $assert_session->selectExists('Theme')
            ->selectOption('claro');
        $assert_session->assertWaitOnAjaxRequest();
        $assert_session->selectExists('Region')
            ->selectOption('pre_content');
        // Switch to a theme that doesn't contain the region selected above.
        $assert_session->selectExists('Theme')
            ->selectOption('stark');
        $assert_session->assertWaitOnAjaxRequest();
        $assert_session->pageTextNotContains('The submitted value Pre-content in the Region element is not allowed.');
        $assert_session->optionExists('Region', '- Select -');
    }

}