Comment # 129 on bug 1112824 from Dead Mozay

(In reply to Wolfgang Rosenauer from comment #128)
> XDG_SESSION_TYPE is "x11"
> wolfgang@ox:~> ps ax | grep X
>  1834 tty7     Sl+    0:00 /usr/bin/Xwayland :1024 -rootless -terminate
> -accessx -core -auth /run/user/464/mutter/Xauthority -listen 4 -listen 5
> -displayfd 6
>  3393 tty2     Sl+    0:02 /usr/bin/X vt2 -displayfd 3 -auth
> /run/user/1000/gdm/Xauthority -background none -noreset -keeptty -verbose 3
> wolfgang@ox:~> grep -i wayland /etc/gdm/custom.conf 
> #WaylandEnable=false
> 
> What is the verdict for that output?
> Wayland or Xorg? I guess "wayland" but XDG_SESSION_TYPE does not reflect
> that.

you need to edit the file /etc/gdm/custom.conf, remove comment from the line
WaylandEnable=false and restart session.