Comment # 5 on bug 79223 from Michel Dänzer

(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.