@Override
protected void beforeCreateWidget() {
super.beforeCreateWidget();
final EventListenerDispatcher dispatcher = this.getEventListenerDispatcher();
dispatcher.addMouseEventListener(new MouseEventAdapter() {
public void onMouseDown(final MouseDownEvent event) {
Viewport.this.onMouseDown(event);
}
});
dispatcher.prepareListenerCollections(EventBitMaskConstants.CHANGE);