I have two monitors in a |- layout (1080x1920 on the left, 2560x1440 on the right). This is configured in xorg.conf. The geometry of monitor 1 is 2560x1440+1080+240 .. note how the monitor 1 is placed 240 pixels down. This is so that the monitors, which are physically in a T-shape (rotated left), match reasonably well when the mouse cursor moves from one to the other. As a consequence, there's a "hole" in my desktop; 1080,0 - 3640,239. When I choose logout, the desktop is displayed in a faded-out manner. But this doesn't take the "hole" into account, so I get: - on the left monitor (1080x1920+0+0), the faded image of what is (was) displayed on that monitor - on the right monitor (2560x1440+1080+240), the faded image of 2560,0 - 3640-1440 .. so that contains the hole, and it looks like the screen image has "jumped down" by 240 pixels.
There was a patch to kwin merged this morning, can you retest. Also can you add a screenshot as that text was confusing to parse. You should be able to use spectacle on a timer.
Created attachment 112706 [details] Desktop in normal use
Created attachment 112707 [details] Desktop during logout This is while loggout out, with the desktop still showing the same applications, in the same location, as the previous screenshot.
The desktop is shaped like a T which has been knocked over, like |-. In the first screenshot, you can see the whole desktop (all monitors), with black bars (where there is no monitor) above and below the right-hand part. In the logout screenshot, everything is dimmed, but you can see that in the right-hand part, the image of the windows has moved down; the black bar previously "above" the monitor is now shown on the monitor, effectively moving the rest down. I'll re-test with latest kwin shortly.
Re-built kdecoration and then kwin, master in both cases. I see the logout-display is now a blur instead of straight darkening. And it's in the right place, on both monitors. Marking as fixed for whatever release is next .. 5.13, I guess.