code: @Override
  	public void onMouseUp(MouseUpEvent event) {
  		// handle click if any?
  		if (dragging) {
  			// shift or CTRL is used when depressed either at beginning or end:
  			shiftOrCtrl = (event.isShiftKeyDown() || event.isControlKeyDown());
  
  			if (timestamp + clickTimeout > new Date().getTime()
  					&& (bounds.getWidth() < pixelTolerance || bounds.getHeight() < pixelTolerance)) {
  				stopDragging();
  				// click behavior instead of drag
  				ToggleSelectionAction action = new ToggleSelectionAction(mapWidget, priorityToSelectedLayer,
  						pixelTolerance);
  				action.toggle(getScreenPosition(event), !shiftOrCtrl);
  				return;
  			}
  		}
  		// normal "rectangle" handling
  		super.onMouseUp(event);
  	}
docs: First mouse button: publish a toggle event on the selection topic for the referenced MapWidget. Second mouse
  button opens a menu.
  
  @param event
  event