@Override
    protected void onPreviewNativeEvent(final NativePreviewEvent event) {
        // We need to preventDefault() on mouseDown events (outside of the
        // DialogBox content) to keep text from being selected when it
        // is dragged.
        final NativeEvent nativeEvent = event.getNativeEvent();

        if (!event.isCanceled() && (event.getTypeInt() == Event.ONMOUSEDOWN)
                && (isCaptionEvent(nativeEvent) || isResizeHandleEvent(nativeEvent))) {
            nativeEvent.preventDefault();
        }

        super.onPreviewNativeEvent(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