i bought a third monitor yesterday and after hours of frustration, trial and error and a lot of caffeine, I finally got my triple-head setup working.
I'm using 2x nVidia GT630 and 3 fullHD 22" screens (and archlinux).
Because of the missing multiple-GPU support in XRandR, I had to use xinerama.
Now X11 recognizes all three monitors perfectly, but KDE doesn't.
Here's what my desktop looks: http://0wx.de/993c65dd0dfe67ddcdc26dfb36c8733c.png (linked, not embedded, because it's a 5760x1080 screenshot)
Moving windows to the right screen is working without any problem.
But the plasma desktop is working on the left and the middle screen only.
I can't set a wallpaper, I cant setup a panel or a plasmoid, i can't even right click at an empty space on the right screen and get a menu.
If i toggle maximize on a window on the right screen, it just maximizes on the middle screen. Also if i play a video in Kaffeine, Dragon Player or VLC and toggle fullscreen, fullscreen enables on the middle screen or the left screen only. If i toggle fullscreen on a window on the right monitor, it just shows in fullscreen on the middle one.
As you can see in the screenshot, the monitor setup in the KDE systemsettings dialog isn't working, because of the missing XRandR i mentioned before. (It's in german on my screenshot, but it should be obvious, that it's not working.)
I also tried removing my ~/.kde4 folder to see if the third monitor would be recognized automatically, after it was set up for 2 monitors before, but this didn't work.
I'm wondering, if theres any config file in ~/.kde4/share/config, where i could set up my third monitor, so it's usable by plasma-desktop.
Actually, I have a similar problem, my screen looks almost identical to your picture there, difference is that I am using Kubuntu rather than arch and get that result, and that I am using an ATI HD6870 with three 1680x1050 monitors.
I found that MATE and XFCE work brilliantly with the triple head setup, KDE sort-of (same as yours) and Cinnamon not at all.
I don't suppose you have had any luck with this setup have you?
I had this problem before:
Since nobody was able to help me, I tried to use awesome as WM in KDE.
With awesomes full xinerama support, maximizing and fullscreen on the third screen works perfectly.
But if i run kaffeine in awesome, I got heavy tearing during video playback, which doesn't happen when using KWin as WM.
Same problem here. This seems to be due to the implementation of Awesome WM itself.
If you don't require composition, you can solve this by simply disabling the composite extention of Xorg. You will loose features like transparency and stuff, but the tearing will be gone.
Other WMs like compiz dont suffer from this problem... but they're just not that awesome..
Wayland will fix this...
Last edited by Archdove (2013-04-11 19:42:23)