In XFCE when one turns on the compositor with a modern graphics card without "sync to vblank" on, there is diagonal tearing whenever there is a quick change on the screen (such as minimizing, maximizing, or restoring a window). There should be a checkbox to turn sync to vblank on or at least a hint box to let the user know the should turn it on in the appropriate place if they have an affected card.
I was not aware that this was an issue, and I have a affected graphics card (an nvidia 7000 series card) that has tearing issues. It took quite a bit of googling to determine that the tearing was due to xfwm4's compositor (some people had similar issues in compiz and other compositors), and that I should toggle the "sync to vblank" option in nvidia-settings.
I am not aware on any Xlib API that allows this from a regular application (We don't use OpenGL at all).
I believe this is something that must be addressed at the driver level, either from the xorg.conf or from any proprietary tool that comes with your hardware, but I don't see how this could be coded in the WM.
After a little bit of investigation, I don't think this is an issue with sync to vblank not being on at all. I'm not really sure what the issue is and I'm not sure where to start with the diagnosis, are you in the IRC channel where I can get some help figuring out where this bug lies? (I'd really like to understand more about how XFCE works as I have some big plans for next year, and experience is always a plus!)
Not an issue.