add: if (!this.checkCanDrop(aEvent, aDragDropObserver)) return; 
context:   dragExit: function (aEvent, aDragDropObserver)    {      if ("onDragExit" in aDragDropObserver)        aDragDropObserver.onDragExit(aEvent, this.mDragSession);    },  
meta: 9421	/local1/tlutelli/issta_data/temp/all_js/javascript/2006_temp/2006/9421/73984e3294b2bb7076551497238f998de25ec5a4/nsDragAndDrop.js/clean/xpfe/global/resources/content/nsDragAndDrop.js