code: protected void handleScroll(ScrollEvent event) {
  
          String styleName = getState().getStyleName();
          int scrollBarrier = getState().getScrollBarrier();
          int barrierMargin = getState().getBarrierMargin();
          if ((m_widget != null) && (scrollBarrier > 0) && (styleName != null)) {
              if (m_widget.getElement().getScrollTop() > (scrollBarrier + barrierMargin)) {
                  m_widget.addStyleDependentName(styleName);
                  m_widget.addStyleName(styleName);
              } else if (m_widget.getElement().getScrollTop() < (scrollBarrier - barrierMargin)) {
                  m_widget.removeStyleDependentName(styleName);
                  m_widget.removeStyleName(styleName);
              }
          }
      }
docs: Handles the scroll event.<p>
  
  @param event the scroll event