At some point in the past couple of weeks I guess my laptop updated libraries or something, because when I had to reboot my laptop yesterday I started seeing massive screen corruption in some applications. It manifested itself in horizontal white/black lines remaining especially when selecting text. This was especially visible in konsole. I couldn’t see any package in particular which had updated recently and I tried looking at a few different Xorg or kernel options but to no avail.

Eventually I looked at the KDE compositor settings. I noticed that the “Rendering Backend” was set to XRender, which as far as I understand it is very old. Updating it to ‘OpenGL 3.1 instantly fixed the issue.

I’m leaving this mostly as a note to myself for if it happens again in the future, but at the same time perhaps it is a wider regression in the ubuntu 20.04 KDE packages so it would help someone else.