When option "Synchronize drawing to the vertical blank" is checked, vsync is on and there is no tearing when moving windows around (with xf86-video-ati driver).
But if the session is locked with light-locker and then unlocked, vsync no longer works (visible tearing effects when dragging windows).
Disactivating then activating the compositor makes it work again.
I'm using Xfce 4.12 and light-locker 1.6.0.
Should I provide any attachment?
Vsync *cannot* work, no way it can do anything. Only way to achieve reliable Vsync is via OpenGL or possibly the Present extension, but current code, no way it will ever work...
Information related to this issue can be found in bug 11642.
*** This bug has been marked as a duplicate of bug 11642 ***