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I tried searching whether similar issue was already discussed on the forums, but didn't find anything; sorry if I missed it.
I have two different systems with Nvidia hardware, one with a 9500M GS, and another one with a GTX 580. After updating to KDE 4.11 (as far as I remember, this issue started appearing at about that time; now it still happens with KDE 4.12) compositing in KDE sometimes doesn't work. By "sometimes" I mean that once in a while I boot the computer and compositing isn't working, then I reboot and everything is fine. Sometimes hitting Alt+Shift+F12 twice helps, but sometimes it doesn't. I didn't gather statistics, but it seems to happen on the 9500M GS system more often than on the GTX580 one. On the former it happens once in about three boots, while on the latter it's something like once per ten boots, but I may be wrong.
I use proprietary driver, since I sometimes use the computer for gaming, and the opensource driver is still not quite up to the task. Btw, games work fine even when compositing doesn't work.
I haven't noticed any similar issue on my other machine with Intel graphics.
If anyone could help in identifying the cause, I would be much obliged, because I have no idea where to start searching.
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