I have six questions:
1. How are top-level windows and sub-surface windows kept synchronized, perhaps using flash in a web-browser as an example?
2. What happens when part of a sub-surface window is obscured within a top-level window, such as using the scroll bar to move the flash animation above or below the top of the window?
3. I assume sub-surfaces have to be part of another window, but can they be nested (i.e. a sub-surface window being part of another sub-surface window), or can sub-surface windows only be part of top-level windows?
4. Do sub-surface windows have complete control over their own buffer, or can top-level windows manipulate one of its sub-surface window buffers before passing it to the compositor?
5. Why is the coordinate counter 31 bits? That seems like a strange number.
6. Is the coordinate counter count the total number of pixels, or the pixel along a particular axis? This isn't clear from the description.