private void endDragging(final MouseUpEvent event) {
        dragging = false;
        DOM.releaseCapture(getElement());

        getDragGlass().hide();
    }
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/Dialog.java