code: public InputListener getDefaultInputListener (final int mouseButton) {
  		if (defaultInputListener == null) {
  			defaultInputListener = new InputListener() {
  				@Override
  				public boolean touchDown (InputEvent event, float x, float y, int pointer, int button) {
  					return true;
  				}
  
  				@Override
  				public void touchUp (InputEvent event, float x, float y, int pointer, int button) {
  					if (event.getButton() == mouseButton)
  						showMenu(event.getStage(), event.getStageX(), event.getStageY());
  				}
  			};
  		}
  
  		return defaultInputListener;
  	}
docs: Returns input listener that can be added to scene2d actor. When mouse button is pressed on that actor,
  menu will be displayed
  @param mouseButton from {@link Buttons}