Sorry for taking so long answering to your last reply,
I'm pretty sure that Xinerama is not activated, as it says in my xorg.conf:
The only thing I use XRandR for is rotating Monitor #3:
Option "Xinerama" "0"
However, I fear there still is a misunderstanding:
Option "RandRRotation" "1"
I do not "move" across screens in Jaunty, at least not as if my several screens acted like one. This means I cannot and I needn't move Windows across the borders.
But there is a thing that works flawlessly in Jaunty, which doesn't in Karmic: There is no way to drag and drop URLs (internet links, file emplacements etc.). To give an example: I'm writing a new email with thunderbird on screen #3 and I want attach a file to it. Under Jaunty I can just open a nautilus window on screen #2. I can not move this window to the other screen. But I can drag and drop the file onto the message window on screen #3. That's what no longer works under Karmic.
I hope to have made clear that it is not a continuous desktop I'm yearning for, but just the possibility to drag and drop URLs. I'm sorry to insist so much but I was under the impression that there's a misunderstanding here, so please excuse me if this further description was unnecessary.
hoping for further ideas