I had this problem, it was terrible. Started dragging windows around to be able to see them
where they were not blocked by that dialog glued onto the front of my screen...
Especially bad when I was working on something which would be lost at logout.
1. I also would lock my screen
2. I had a screensaver chosen from 'configure your desktop'/hardware/display/slideshow.
3. no nvidia, I have a standard builtin intel controller.
4. as you say, nothing short of logging out seemed to help. adding new desktops just got
the same ugly patch on the new ones.
This happened along with various other problems,
and I ended up dumping that install of PCLOS, and
moving to FullMonty on another partition.
At the time I also remember asking here if there was some way to reinitialize the display to
fix this problem, but I don't recall getting any suggestions.
So, I feel for you, but I am sorry I can't help.