public void onMouseDown(MouseDownEvent event) {
   setMouseDown(true);
   updateCSS();
   DOM.setCapture(view.getElement());
   dragStartX = event.getX();
   dragStartY = event.getY();
   event.preventDefault();
 }
 public void onMouseUp(MouseUpEvent event) {
   Element element = view.getElement();
   if (!isDragging()) {
     updateCSS();
     limitToBoundingBox();
     if (NodeProxy.EXPANDED.equals(model.getState())) {
       model.setState(NodeProxy.COLLAPSED);
       for (NodeProxy neighbor : graph.getNeighbors(model)) {
         boolean collapsed = NodeProxy.COLLAPSED.equals(neighbor.getState());
         boolean leafNode = graph.degree(neighbor) == 1;
         if (collapsed && leafNode) {
           graph.removeNode(neighbor);
         }
       }
     }
     parent.toggle(model);
   } else {
     parent.save(model, true);
     setDragging(false);
   }
   setMouseDown(false);
   DOM.releaseCapture(element);
   event.preventDefault();
 }