(In reply to Thomas Rohloff from comment #11)
> I'm now on LXQT with compton as compositor. The tearing is still there.

What command line parameters / configuration are you using for compton?
Something like

 compton --backend glx --glx-swap-method 2 --unredir-if-possible

works well for me.

With current xf86-video-ati Git master, you can also try Option "TearFree",
which should prevent tearing even without a compositing manager. Its downside
is that it disables DRI page flipping ATM.