private void showMarker(final double left, final double top, final double width, final double height) {
        final Rect outer = ElementUtil.getClientRect(scrollPanel.getElement());
        final Rect inner = new Rect(top, top + height, left, left + width);
        final Rect min = Rect.min(outer, inner);
        marker.getElement().removeClassName("flexLayout-marker-tab");
        marker.show(min);
    }
Tests the supplied layout to see if it is the current mouse target and
returns mouse target if it is.

@param x               The mouse x coordinate.
@param y               The mouse y coordinate.
@param splitter        True if the layout is a splitter. Splitters are targeted
                       outside their rect.
@param layoutConfig    The layout data to test to see if it is a target.
@param positionAndSize The position and size of the layout being tested.
@return The mouse target if the tested layout is the target, else null.
stroom-core-client/src/main/java/stroom/dashboard/client/flexlayout/FlexLayout.java