In some occasions, in a multiscreen configuration, one of the two screens may turn up to have a black background and to show no context menu on the desktop. Sometimes switching setup in the settings fixes the problem and the desktop is back to normal. Sometimes this does not work and unplugging and plugging the screen fixes the situation. Sometimes restarting plasmashell with "plasmashell --replace" fixes the situation.
STEPS TO REPRODUCE
Unfortunately this does not happen deterministically, but it is not infrequent. May happen immediately after the boot.
One of the screens is working as usual, the other one has a black background and no context menu can be opened. If this happens on the primary screen, the panel is visible and seems to work as usual. Windows can be moved on the black background.
Background should be present and context menu should open with the usual mouse button.
Linux/KDE Plasma: 5.19.13
KDE Plasma Version: 5.25.5 Wayland
KDE Frameworks Version: 5.98.0
Qt Version: 5.15.6
On the same machine I used to connect a HDMI 1080p screen and I do not remember this problem. This situation began after I bought a 4K USB-C screen. If the screen itself (or the 4K resolution) is really the cause of the issue is unknown. The failing setup is with two 4K screens, one with scaling factor 2 and the other with scaling factor 1.5.
Thank you for the bug report!
Please note that Plasma 5.25.5 is not supported for much longer by KDE; supported versions are 5.24, and 5.26 or newer.
If at all possible please upgrade to a supported version and verify that the bug is still happening there.
Ultimately this is a basically unfixable bug caused by the current mapping system being broken by design. It should be fixed once we move to a better system. See Bug 450068.
*** This bug has been marked as a duplicate of bug 450068 ***
I read the bugreport you linked. Are you sure this is really a duplicate of that? In that report I read: "Wallpaper/tablet settings are restored to default". One thing is losing some custom configuration, one other thing is a malfunctioning system, missing background and missing context menu that can only be fixed by rebooting or restarting plasmashell. Please note that I did not read the entire thread, I'm just asking for a confirmation.
I also sometimes see that the background is restored to default, that is a different, less relevant issue.
It's ultimately the same issue. The system forgot that your screen had a containment on it, so you see nothing, just a black background. You can't right-click on it because that functionality is provided by the Plasma containment, which has gone missing.
Fixing 450068 is the only reliable way to prevent this kind of thing from happening ever again.