code: protected int getPopupHeight() {
  
          if (m_popup.isShowing()) {
              return m_popup.getOffsetHeight();
          } else {
              Element el = CmsDomUtil.clone(m_popup.getElement());
              RootPanel.getBodyElement().appendChild(el);
              el.getStyle().setProperty("position", "absolute");
              int height = el.getOffsetHeight();
              el.removeFromParent();
              return height;
          }
      }
docs: Returns the offset height of the popup panel, should also work when the popup is currently not showing.<p>
  
  @return the offset height