flex中拖拽 接收容器必须设backgroundcolor

之前代码

<mx:Canvas?id="drawSpace" width="500" height="200" horizontalCenter="0" verticalCenter="0" dragEnter="canvasDragEnterHandler(event)" dragDrop="canvasDragHandler(event)">
??<mx:Box width="20" height="20" backgroundColor="0x00FFCC" x="137" y="66" mouseDown="boxMouseDownHandler(event);">
???
??</mx:Box>
?</mx:Canvas>

发现drawSpace怎样都无法接收box组件，修改成下面这样，加了个背景颜色后可以了

<mx:Canvas backgroundColor="0xEEEEEE" width="500" height="200" horizontalCenter="0" verticalCenter="0" dragEnter="canvasDragEnterHandler(event)" dragDrop="canvasDragHandler(event)">
??<mx:Box width="20" height="20" backgroundColor="0x00FFCC" x="137" y="66" mouseDown="boxMouseDownHandler(event);">
???
??</mx:Box>
?</mx:Canvas>

这算什么，bug？。。。。