private int getRelativeY(final Event e, Element target) {
        return e.getClientY() - target.getAbsoluteTop() + target.getScrollTop() +
                target.getOwnerDocument().getScrollTop();
    }
Gets the mouse y-position relative to a given element.

@param target the element whose coordinate system is to be used
@return the relative y-position
stroom-core-client-widget/src/main/java/stroom/widget/button/client/Button.java