code: @Override
  	public void onDown(HumanInputEvent<?> event) {
  		if (dragging && leftWidget) {
  			// mouse was moved outside of widget
  			doSelect(event);
  
  		} else if (!isRightMouseButton(event)) {
  			// no point trying to select when there is no active layer
  			dragging = true;
  			leftWidget = false;
  			timestamp = new Date().getTime();
  			begin = getLocation(event, RenderSpace.SCREEN);
  			bounds = new Bbox(begin.getX(), begin.getY(), 0.0, 0.0);
  			shift = event.isShiftKeyDown();
  			rectangle = new Rectangle("selectionRectangle");
  			rectangle.setStyle(rectangleStyle);
  			rectangle.setBounds(bounds);
  			mapWidget.render(rectangle, RenderGroup.SCREEN, RenderStatus.UPDATE);
  		}
  	}
docs: Start dragging, register base for selection rectangle.
  
  @param event
  event