@Override
        public void onMouseMove(final MouseMoveEvent event) {
            continueDragging(event);
        }
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