> How can you say that you cannot narrow down the possible culprits?

Because the .tar.gz of the qt4.5 source is 100 MB in size. If we had a crash report saying "there was a segmentation fault right here," we could maybe figure out where in that beast of a source code it was -- but just asking "what line in the KDE 4.2 source is calling the wrong function in the Qt 4.5 source which is also misusing EXA when it repaints the screen" is a question that can't be answered without much more information than the problem itself provides.

> So, waiting for an yet unstable alpha release is the solution? 

I'm not saying that you should jump on the Karmic alpha -- once people have solid Karmic-4.3 integration, there will almost surely be a .deb posted to the jaunty-backports repository.

> How about proposing KDE 4.2 compiled against qt4.4 (shipped by default) and package qt4.5 as an alternative with a BIG FAT WARNING? I haven't seen anything preventing such a strategy on a release-update. 

You can get KDE 4.2 + Qt 4.4 + Kubuntu! However, you will have to use Intrepid to do it. As jaunty-backports will probably eventually contain KDE 4.3, so too the intrepid-backports repository contains, right now, KDE 4.2. And Intrepid has not been retrofitted to use Qt 4.5 -- 4.5 is strictly Jaunty and up.

> According to this changelog this bug might have already been solved in qt.

I don't see anything in that changelog to suggest a fix for anything EXA-specific, much less rendering artifacts. What line are you referring to, specifically? (I'm not really a Kubuntu dev, so don't take my word as gospel here. The Debian unstable repository (sid) contains a .deb for Qt 4.5.1, and ScottK on IRC says that the current plan is to merge this into Karmic, but keep it out of Jaunty. It might get backported later; I don't know.)

> Which KDE bug are you referring to?

I was referring to this comment (comment #21) by Aaron J. Seigo:

https://bugs.kde.org/show_bug.cgi?id=187356#c21

> Are you referring to the KDE 4.3 fix? What if the bug is really in Qt 4.5?

I was referring to using UXA. (More specifically, since it looks like nobody has placed these instructions explicitly here, adding the line [    Option    "AccelMethod"    "UXA"] to the "Device" section in /etc/X11/xorg.conf .) So far UXA has been working for me, but the problem was much less prevalent for me than it apparently is for you.

>using a "better" version of Qt within Jaunty (perhaps using an optional repository?)

Jaunty has already shipped with Qt 4.5, and my amateurish attempts to simulate a downgrade to Qt 4.4 with aptitude and the Intrepid repositories have not looked promising. I don't think anyone can provide you with a PPA (an optional repository) that would solve the multitude of dependencies. Everything (?) in Kubuntu Jaunty was built with the idea, "okay, we're releasing a Qt 4.5 operating system this April." Much of it breaks when you try to pull the tablecloth out from under it. 

If you want KDE 4.2 + Kubuntu, and you cannot enable UXA for some reason or another (or if UXA doesn't solve the problem), then it looks like your only option is Intrepid at the moment. Karmic may offer a backport of KDE 4.3 in a couple weeks if you're willing to wait.