code: public function renderItems()
      {
          $items = [];
          $index = 0;
          foreach ($this->items as $header => $item) {
              $options = ArrayHelper::getValue($item, 'options', []);            
              Html::addCssClass($options, ($this->box) ? 'panel box' : 'panel' );
              $items[] = Html::tag('div', $this->renderItem($header, $item, ++$index), $options);
          }
  
          return implode("\n", $items);
      }
docs: Renders collapsible items as specified on [[items]].
  @return string the rendering result