code: protected void shiftDown(int shift) {
  
          int max = getMaximumVerticalScrollPosition();
          if ((m_currentValue + shift) < max) {
              setVerticalScrollPosition(m_currentValue + shift);
          } else {
              setVerticalScrollPosition(max);
          }
      }
docs: Shifts the scroll position down.<p>
  
  @param shift the shift size