code: protected void onMouseWheelScroll(Event event) {
  
          int scrollStep = event.getMouseWheelVelocityY() * 5;
          Element scrollTarget;
          if (getCurrentTarget() != null) {
              scrollTarget = getCurrentTarget().getElement();
          } else {
              scrollTarget = RootPanel.getBodyElement();
          }
          while ((scrollTarget.getScrollHeight() == scrollTarget.getClientHeight())
              && (scrollTarget != RootPanel.getBodyElement())) {
              scrollTarget = scrollTarget.getParentElement();
          }
          if (scrollTarget == RootPanel.getBodyElement()) {
              int top = Window.getScrollTop() + scrollStep;
              int left = Window.getScrollLeft();
              if (top < 0) {
                  top = 0;
              }
              Window.scrollTo(left, top);
          } else {
              int top = scrollTarget.getScrollTop() + scrollStep;
              if (top < 0) {
                  top = 0;
              }
              scrollTarget.setScrollTop(top);
          }
          onMove(event);
      }
docs: Execute on mouse wheel event.<p>
  
  @param event the native event