I usually use my laptop connected to an external monitor. When I disconnect it, as expected, a dialog comes up to reconfigure my display settings.

So far so good, I set the resolution of the notebook display, hit OK, and it changes the resolution. But, the desktop does not resize accordingly, so it's wider and larger than the current resolution, eg. the bottom and right part of the desktop are outside what I can see on my notebook monitor.

I have to logout and back in to get a desktop of the right size.

I'm running KDE 4.8.4, Opensuse 12.1 Tumbleweed

What happens to maximized windows? Do they exceed the screen geometry as well?

Yes, maximized windows exceed the geometry. Tiling windows too.

It behaves like the desktop is still the size of the external monitor.

Checked "xrandr -q" whether the geometry actually changed?

I'll check. Most likely, it didn't change.

What should I do in that case?

Try setting it using xrandr.
If that works, there's an issue with krandr (geometry detection, libXrandr usage, whatever)
If not, there's an issue with libXrandr.

Nvidia GPU? In case what driver version?

No, it's an Intel HD card.

According to lspci:
00:02.0 VGA compatible controller: Intel Corporation 3rd Gen Core processor Graphics Controller (rev 09)

luebking wrote:Try setting it using xrandr.
If that works, there's an issue with krandr (geometry detection, libXrandr usage, whatever)
If not, there's an issue with libXrandr.

Nvidia GPU? In case what driver version?

I ran xrandr, and the geometry was fine. 1366x768.
That's what it's supposed to be in the laptop display.

Does it make a difference how I setup the displays when both monitors are connected?

I have it like this:

LDVS1 (laptop monitor)
Size: 1366 x 768
Position: Absolute 0 0

VGA1 (external LCD)
Size: 1920 x 1080
Position: Clone of LDVS1

Primary output is LDVS1

Does that mean "i ran xrandr -s 1366x768 and everything works as expected" or "xrandr -q says 1366x768 but the desktop and maximized windows still have the wrong size"
Also nb. that "xrandr -q" easily tricks the innocent with a panning canvas (ie. the resolution is whatever but you're supposed to see only a part of it) - in that case the screen will shift with the mouse cursor.

If xrandr says "correct dims for sure" but it does not work, you should run

xev -root | grep -EA4 '^RR'

to ensure the resolution change is communicated.
If there's no relevant output (you'll notice...) there's a bug in either libxrandr or the server.

"xrandr -q says 1366x768 but the desktop and maximized windows still have the wrong size"

but it doesn't shift with the cursor.

Since at least to applications don't receive the resolution change event, you should check the root events then.
If there are such events, it's a bug in Qt (which should emit signals whenever the desktop/screen is resized)