code: public void addRequestPanelDisplayedMessageChangedListener(DisplayedMessageChangedListener messageChangedListener){
          if (requestPanelDisplayedMessageChangedListener == null) {
              requestPanelDisplayedMessageChangedListener = createList();
          }
          requestPanelDisplayedMessageChangedListener.add(messageChangedListener);
      }
docs: Adds the given {@link DisplayedMessageChangedListener} to the view hook, to be later added to the
  the {@link org.parosproxy.paros.view.View}s {@link org.zaproxy.zap.extension.httppanel.HttpPanelRequest} ChangeListeners.
  
  @see org.zaproxy.zap.extension.httppanel.HttpPanelRequest#addDisplayedMessageChangedListener(DisplayedMessageChangedListener)
  @param messageChangedListener the listener for the request panel.
  @since TODO add version