what happens is I start xdm and it is in cloned mode, next I log in and set the dual-head with xrandr which results in the artefacts on the second screen (actually DVI-I_1/digital, but since it is rightmost I call it second).
To get rid of the artifacts I kill kwin and start fvwm (twm and openbox don't help only fvwm) then the artifact is gone and I can kill fvwm and go back to kwin all the while there is no new events in xorg.0.log since going from clone to dual-head.
This even happens if I have the second screen overlap that means both are 1280x1024 and screen two is now at offest 640x0 then there is that strip of 640x1024 which is rendered correctly on screen 1 but not on screen 2.
Created attachment 12885 [details]
camera screenshot of the middle where both screens meet
Created attachment 12886 [details]
screengrab screenshot of the desktop
The effect on the external display looks exactly like in my bug #13474. I use a cloned screen though...
Probably a duplicate?
screw fvwm all I need to do is switch to the console (which is actually another card!) and come back to get it pretty. Looks like bug #13474.
*** This bug has been marked as a duplicate of bug 13474 ***