This is a screenshot when the problem arose. After reboot everything was back to normal!
http://filippides.eu/test/20190311.png
It’s probably a bad idea to update from a Konsole window while logged in to KDE.
Better to log out of the (KDE) session, switch to a virtual terminal (say Terminal #2 Ctrl-Alt-F2), login in there and then execute zypper dup.
FWIW: I’ve been running TW since it started, always upgraded from KDE ( yakuake as a terminal emulator ), never had issues. As we can see from the pic, the actual install of the packages has not yet begun…
Obviously the problem is not the update, but the Plasma.
Something went wrong BEFORE I gave the zypper command, probably with the cache memory of the plasma screen that mixed with memory of Thunderbird as it loaded to RAM!
That’s the way I used to update — until the desktop crashed during an update. Desktop crashes are probably quite rare, but since then I’ve always logged out from the KDE session, switched to a VT and ran zypper dup from there, I also always reboot after any zypper dup regardless of whether it’s needed.
As we can see from the pic, the actual install of the packages has not yet begun…
Yes, indeed. I should have made clear, I wasn’t suggesting the update method was the cause of the screen corruption, merely, IMHO, suggesting it wasn’t perhaps the “best practice”. However you’ve not had problems with updating from a terminal emulator, and I expect many people choose that method. 
Hey Paul,
I think that most laptop users don’t make their wifi connection available to other users on the system ( most only have one user ), which means that they lose their network connection after logging out. On a desktop system, with wired connection, that’s not the case, and although I think it’s not needed, yours is the most safest practice.
BTW I did see plasma crashing once, when installing a Mesa* update ( found some old notes ), but that should not have happened ( a bug was filed at the time, ~2 years ago ).
Nevertheless the problem is that the Plasma crashed, not the update. 
Does anyone have a clue?
Do you need more data?
Can anybody fix that?
Unless it’s repeatable I doubt there’s too much you can do at this stage.
I’ve seen, extremely rarely, screen corruption vaguely similar to that shown when using the nouveau driver with KDE. Never worried about it myself, “odd things happen”…
If it happens again you could try toggling compositing on/off (Alt-Shift-F12), or log out/in to KDE, a full reboot may perhaps not have been necessary.
I think only if it happens consistently, or you are able to “trigger” the corruption, will it perhaps be possible to find the cause.
I usually update from a terminal emulatore (“xterm”) in a KDE session. But I run the update inside a “screen” session. So even if the desktop crashes, the update continues.
However, yes, I probably should switch to a VT. I do run into other problems because of updating within KDE. Sometime a major KDE update will cause problems for the desktop session, such as shutdown doesn’t work.
I know that “odd thinks happen” (bugs) from time to time, so I wrote this in order to stop them from happening 
clue:
Thunderbird starts in 4th screen and this time I opened it when the DSL was down and connecting. Just then and because I waited some time, I clicked on Thunderbird. Clicking on system tray caused the problem.
I think that this is not repeatable, but will help plasma programmers.
Thank you all for your answers.
Filippos
Probably better to submit a bug report Log in to KDE Bugtracking System - I’m not too sure against what component, plasmashell maybe, which is a good catch-all for bugs in plasma5. Although, as I wrote earlier, unless it’s repeatable I doubt much can be done.