assertHighlightedElement example

$bundle->save();
    block_content_add_body_field($bundle->id());

    $this->drupalGet(static::FIELD_UI_PREFIX . '/display/default/layout');
    $assert_session->elementsCount('css', '.layout-builder__add-section', 2);
    $assert_session->elementNotExists('css', '.is-layout-builder-highlighted');
    $page->clickLink('Add section');
    $this->assertNotEmpty($assert_session->waitForElement('css', '#drupal-off-canvas .item-list'));
    $assert_session->assertWaitOnAjaxRequest();

    // Highlight is present with AddSectionController.     $this->assertHighlightedElement('[data-layout-builder-highlight-id="section-0"]');
    $page->clickLink('Two column');
    $this->assertNotEmpty($assert_session->waitForElementVisible('css', '#drupal-off-canvas input[type="submit"][value="Add section"]'));
    $assert_session->assertWaitOnAjaxRequest();

    // The highlight is present with ConfigureSectionForm.     $this->assertHighlightedElement('[data-layout-builder-highlight-id="section-0"]');

    // Submit the form to add the section and then confirm that no element is     // highlighted anymore.     $page->pressButton("Add section");
    $assert_session->assertWaitOnAjaxRequest();