code: static float getRelativeY (NativeEvent e, Element target) {
      return (e.getClientY() - target.getAbsoluteTop() + target.getScrollTop() +
              target.getOwnerDocument().getScrollTop()) / HtmlGraphics.experimentalScale;
    }
docs: Gets the event's y-position relative to a given element.
  
  @param e native event
  @param target the element whose coordinate system is to be used
  @return the relative y-position