AVM2 bug when rolling over movieclips

When making examples for a recent workshop, I stumbled over a bug in the Actionscript Virtual Machine used for Flash Player 9 and higher (AVM2). If you are using a movieclip inside a button, the hit area won’t work as it should. When moving the mouse across the button, it will constantly blink giving a very flimsy impression. You can test this in the example below.

Here’s source files that show the bug and the fix FLA - SWF - TwoStateButton.as

Switching the movieclip to graphic fixes the issue, but why won’t this work? It’s always worked well before, so this must be a bug in AVM2? Another thing is that if you hover this button for a few seconds, the Flash IDE crashes instantly…

If you’re unable to repoduce the bug - feel free to check this video that shows it and please post a comment telling what Flash Player version/browser you are using. I’ve been able to reproduce the bug on all OSX browsers + IE8 on Win7.

Submitted this as a bug in the bugbase https://bugs.adobe.com/jira/browse/FP-4396