Comment 4 for bug 1120077

I have performed additional troubleshooting and was able to find a workaround.

My configuration is as follows:
Ubuntu 13.04 (Raring), 3 way multiseat, Gigabyte GA-990FXA-UD3, 2 x GeForce GTS 450 (also used as audio for seats 1 and 2), 1 x GeForce GT 440 (also used as audio for seat 3), .
Config files: http://pastebin.com/u/bastafidli

The following kdmrc file causes the issue:
http://pastebin.com/qa6uHZ7M
due to the following line

ServerCmd=/usr/bin/X0 -br -audit 0 -nolisten tcp -layout seat0 -sharevts -novtswitch -isolateDevice PCI:1:0:0 -keeptty

The following kdmrc file solves the issue:
http://pastebin.com/bxGbHNra
by changing the line like this
# No -sharevts due to https://bugs.launchpad.net/ubuntu/+source/kde-runtime/+bug/1120077
ServerCmd=/usr/bin/X0 -br -audit 0 -nolisten tcp -layout seat0 -novtswitch -isolateDevice PCI:1:0:0 -keeptty

The problem is described here
https://wiki.archlinux.org/index.php/Xorg_multiseat#Little_black_boxes.2Fdots_on_the_desktop

For me it can be only reproduced if there are 3 seats and it always happens only on the first graphics card (not necessarily first seats). There are the things I have tried when troubleshooting:
- I have swapped graphics cards for seat0 and seat 1 in xorg.conf. Seat 0 had second graphics card and Seat 1 had first graphics card. Seat 0 still had artifacts.
- I have modified kdmrc and disabled [X-:1-Core] and [X-:2-Core] sections to launch only 1 seat. The problem went away
- I have modified kdmrc and disabled [X-:3-Core] section to launch only 2 seats. The problem went away
- With 2 seats configured in kdmrc I have tried various combinations of 2 graphics cards for seat 0 and seat 1. The problem went away.

The only way for me to reproduce the problem is to have 3 seats running and the seat which uses the first graphics card has "-sharevts" option in kdmrc.