Here's a screenshot of what happens after I start KDE normally with normal
orientation and then use xrandr or the KDE desktop rotate feature in the control
panel.
What is not apparent in the screenshot is that kwin at least thinks that the
desktop extends to right, off-screen. This seems generally universal: the
desktop background is clipped, and the trashcan + system tray is not visible at
right. But it is definitely most apparent when I move windows around.
If I start KDE with the rotated setting enabled at startup, it seems to come up
correctly. Clearly something in KDE should just be told that screen physical
dimensions changed somehow but what and where? That's the question...
I've tried to fool the system to take orientation into account by switching the
display sizes with xrandr but it's of no use, either. Resolution goes down but
the black bottom border remains and display dimensions remain wrong just as well.
Created an attachment (id=2006)
Screenshot with a konsole placed strategically