https://bugs.freedesktop.org/show_bug.cgi?id=79223
--- Comment #5 from Michel Dänzer michel@daenzer.net --- (In reply to comment #4)
I'm also seeing something else waiting for vsync. With glReadPixels() out of the picture, and glXSwapIntervalMESA(0), I'm still getting 60 fps and no tearing. Possibly related?
Either
Option "SwapbuffersWait" "off"
and/or
Option "EnablePageFlip" "off"
should avoid that.