[flexcoders] Child Panel can be dragged out of its parent Canvas

[Mic]

When adding child Panels to a Canvas with actionscript, the Panel
cannot be dragged out of the Canvas at top or left, but can be
dragged (made to disappear) at right and bottom of Canvas. Not sure
why the child is not constrained within its parent? Not sure whether
this is relevant but there is a Module as a parent:

<mx:Module
   var sPanel:SuperPanel;
   target = this.canvas1;
   this.target.addChild(sPanel);

   <mx:Canvas canvas1

TIA Mic.