GPU Hardware Acceleration + setMask()

For the life of me, I cannot get GPU hardware acceleration to coexist with a mask in the same Flash file.

I can pull in an H.264 video, and enable the GPU Hardware Acceleration without any trouble. The video plays fine, and is clearly taking advantage of all the extra power from the graphics card.

BUT if I add some dynamic text to the Stage, and mask part of it out, it looks as if the GPU acceleration is switched off as soon as I apply the mask.

I’ve enclosed an example of what’s going on, with the mask getting switched on and off in the Flash file. After viewing the first few seconds (via the enclosed HTML file in a browser), you should be able to see the video going from being aliased, to GPU accelerated smoothness.

Is this a bug with Flash 10? Am I doing something wrong in my code? …or is this functionality simply not supported?

Here is the file: http://www.filefactory.com/file/afh9fga/n/h264_setMask_example_zip

Thanks for any help.