docs: public void cleanUpContainers() {
  
          List<String> removed = new ArrayList<String>();
          for (Entry<String, CmsContainerPageContainer> entry : m_targetContainers.entrySet()) {
              if (!RootPanel.getBodyElement().isOrHasChild(entry.getValue().getElement())) {
                  removed.add(entry.getKey());
              }
          }
          for (String containerId : removed) {
              m_targetContainers.remove(containerId);
              m_containers.remove(containerId);
          }
          if (removed.size() > 0) {
              scheduleGalleryUpdate();
          }
      }