Flash: Debugging unexplained sprites?

I've been trying to debug a strange white rectangle that is appearing in my Flash application:

alt text http://img.skitch.com/20100813-tck3gimp3mhe9p1nssw26b1kdy.png

The standard debugging tools — binary search, breakpoints, hg bisect — have lead to dead ends.

Any suggestions tools I could use to debug this? Is there something like Firebug's inspector, which lets you mouse over HTML DOM elements to get informat开发者_高级运维ion about them?

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Don't the mouse events bubble as they do in the DOM? Could you not use the Sprite (or lower) Prototype to debug print?