Image corruption with Xrender and fullscreen SDL

[bobo1on1]

I have written a small app that copies the root window in a pixmap using the render extension with bilinear scaling, copies that pixmap to an XImage and copies that XImage to a window on another display connection.
This operation is synchronized to the vertical blank using glXWaitVideoSyncSGI.

When a fullscreen SDL app runs (tested with tuxracer and xmoto), very often only partially rendered scenes are captured.
This does not happen when the application is windowed, or when "Allow Flipping" is disabled.

This is what it's supposed to look like:


This is what it looks like quite often: