code: protected function renderMenu($items, $parentItem)
      {
          $options = ArrayHelper::getValue($parentItem, 'options');
          $label = $this->getLabel($parentItem);
          $items = Html::tag('div', $this->renderItems($items), ['class' => 'menu']);
          Html::addCssClass($options, 'ui');
          Html::addCssClass($options, 'header');
          return
              Html::tag(
                  'div',
                  Html::tag('div', $label, $options) . $items,
                  ['class' => 'item']
              );
      }
docs: Renders the given items as a dropdown.
  This method is called to create sub-menus.
  
  @param array $items the given items. Please refer to [[Dropdown::items]] for the array structure.
  @param array $parentItem the parent item information. Please refer to [[items]] for the structure of this array.
  
  @return string the rendering result.