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