Steps to reproduce:
1. Run this app.
<mx:Application xmlns:mx="
http://www.adobe.com/2006/mxml" layout="absolute" backgroundColor="#ff0000">
<mx:HBox id="controlBar" height="100" width="100%" backgroundColor="#ffff00" horizontalAlign="center">
<mx:Button label="Button"/>
</mx:HBox>
</mx:Application>
Actual Results:
You end up with a red bar equal to the height of the HBox. I found out this is because controlBar is a member of mx:Application.
Expected Results:
I would expect some kind of warning or error on this.
Workaround (if any):