Compositing on Xfce: VSync issues
Unfortuantely Xfce's compositor uses Xrender which does not support vertical synchronization. This seems to create a very constant tearing effect in vsynced OpenGL applications. Some drivers like Intel's open source driver or AMD or NVidia's binary blobs manage to force Xrender to vsync but the open source radeon driver doesn't.
An easy enough way around this is just to use a compositor like compton that uses OpenGL and allows vsync. Just have to install it (from the AUR though), copy the default config to \~/.compton.conf, remove all the nasty animation effects, change the backend to glx and set the vsync option to something like opengl-swc. Now add it to the autostart and compositing can go its way again. Don't forget to disable Xfce's compositor of course.
My thanks go to ViperAFK whose postled me to this nice way of still having compositing with radeon.