I was running KUbuntu 11.10, and one of the standard "apt-get upgrade" updates completely broke Kwin in compositing mode. I upgraded to 12.04 thinking that might fix it, but it's the same there. I see it on three separate machines, so it isn't specific to any hardware.

Symptoms:

* When clicking on windows on the desktop (even plasma things like the KDE menu), the entire desktop becomes unresponsive for about 20 seconds. It will not process keyboard or mouse input. The machine is still up and running; I can ssh into it remotely. The mouse cursor is stuck in one place, the screen appears to be in the middle of rendering things, and the keyboard is unresponsive as well. After about 20 seconds, whatever you did will happen. For example, if I try to drag a window by its title bar, the whole machine "stops" for a while, but finally, the window does move. It's completely unusable though.

* Turning off compositing makes the problem go away. The 2D desktop is OK, but for obscure reasons I won't go into here having to do with the behavior of a particular application under compositing vs non-compositing desktops, I need the compositing mode. Not because of the eye candy, but because it subtly changes the behavior of some 3D applications when they're run in compositing mode.

* Creating a new user with brand new KDE settings does not help. The new user sees the same problem exactly, so it's not my settings. I can't even use it enough to turn off compositing!

* .xsession-error is not proving helpful. Even on a fully working system with a brand new untouched-by-human-hands install, it's clogged up with errors from "nepomuk" and "Soprano". IMHO, this looks very unprofessional, but it also acts to hide any "real" errors that might be there in hundreds of lines of "expected" errors.

* Turning off various compositing effects does not help. I've turned off every single one, and see the same problem. Anyway, they were all working a few days ago.

Any ideas? If it was just one machine, I'd think maybe it was a problem with just that box, but the update broke three different machines.

Today there was an nVidia driver update, and the problem is fixed with that .

Looks like it was a driver problem.

Still, I think this kind of thing is pretty bad. People wonder why Linux has less than 1% desktop market share... when a random daily update can blow your desktop out of the water for a week, regardless of what the guilty software is, it seems rather unacceptable outside of a hobbyist environment. I use Linux exclusively, but in all honesty I cannot recommend it as a desktop to my less technical friends because of issues like this. Today the nVidia driver, yesterday a networking driver, tomorrow the Pulse sound system... the day after that, something else. Always something.

Ah well