SUMMARY After unplugging an external screen, the plasma panel disappeared. Logging out and back in does not help. Looks like I need to re-configure my panel from scratch. STEPS TO REPRODUCE I don't know how to reproduce. All I did is connect an external screen, set it up to mirror the internal screen, set both to a lower resolution, and the unplug the external screen. SOFTWARE/OS VERSIONS Operating System: Debian GNU/Linux KDE Plasma Version: 5.25.4 KDE Frameworks Version: 5.97.0 Qt Version: 5.15.4 Kernel Version: 5.18.0-4-amd64 (64-bit) Graphics Platform: X11 Processors: 8 × Intel® Xeon® CPU E3-1505M v5 @ 2.80GHz Memory: 31,2 GiB of RAM Graphics Processor: Mesa Intel® HD Graphics P530 Manufacturer: LENOVO Product Name: 20ENCTO1WW System Version: ThinkPad P50 ADDITIONAL INFORMATION
Oh that is great, after some more unplugging and replugging, the old panel now sits at the middle of my laptop screen, with the external screen using a different resolution so it sits properly at the bottom there. Somehow it also doesn't remember to switch the resolution properly when I plug in that external screen (something that used to work fine with another external screen until a week ago)... but I guess that is a separate bug.
I can pretty much reproduce losing the panel by setting the external screen to be a "replica of" the internal one, and then unplugging.
*** This bug has been marked as a duplicate of bug 356225 ***