Before I found out that a custom config is needed to solve the problem, I tested the following :
1) downgrade to xf86-video-ati 7.7.0 : it SOLVES THE PROBLEM (without any custom config)
2) patching current kernel radeon driver, "just in case" with
http://www.spinics.net/lists/dri-devel/msg124318.html : it doesn't solve problem
3) upgrading kernel to 4.8.10 : it doesn't solve problem
4) try to force DRI2 : it doesn't solve problem
The only 2 solutions that work (here) are :
1) downgrade to xf86-video-ati 7.7.0
2) using the following config with xf86-video-ati 7.8.0
Code:
Section "Device"
Identifier "Radeon"
Driver "radeon"
#Option "AccelMethod" "glamor"
Option "AccelMethod" "exa"
Option "DRI" "3"
#Option "TearFree" "on"
Option "ColorTiling" "on"
Option "ColorTiling2D" "on"
EndSection
Edit : refined config above..