private boolean isResizeHandleEvent(final NativeEvent event) {
        final EventTarget target = event.getEventTarget();
        if (Element.is(target)) {
            final Element element = Element.as(target);
            return resizeN.getElement().isOrHasChild(element) ||
                    resizeE.getElement().isOrHasChild(element) ||
                    resizeS.getElement().isOrHasChild(element) ||
                    resizeW.getElement().isOrHasChild(element) ||
                    resizeNE.getElement().isOrHasChild(element) ||
                    resizeNW.getElement().isOrHasChild(element) ||
                    resizeSE.getElement().isOrHasChild(element) ||
                    resizeSW.getElement().isOrHasChild(element);
        }
        return false;
    }
Called on mouse up in the caption area, ends dragging by ending event
capture.

@param event the mouse up event that ended dragging
@see DOM#releaseCapture
@see #beginDragging
@see #endDragging
stroom-core-client-widget/src/main/java/stroom/widget/popup/client/view/ResizableDialog.java