It was because of how HW accelerated video decoding worked. You would specify the coordinates of a rectangle on the screen where to play the video. Then you'd d

It was because of how HW accelerated video decoding worked. You would specify the coordinates of a rectangle on the screen where to play the video. Then you'd display a rectangle of a certain color (#010000, I believe), to be used as a mask. The window manager could show windows on top of the video, and the masking would work even though the video decoding HW knew nothing about window management and vice versa.