code: private void onShow() {
  
          if (m_detailsFieldset != null) {
              m_detailsFieldset.getContentPanel().getElement().getStyle().setPropertyPx(
                  "maxHeight",
                  getAvailableHeight(m_messageWidget.getOffsetHeight()));
          }
      }
docs: Checks the available space and sets max-height to the details field-set.