Indeed it looks like a bug. I finally found a solution, though. Check the main post edit for details.

You are right, I should have made this clear. I am not on Nvidia, I am using an old Thinkpad on Intel Haswell.

I’m glad to know my problem isn’t completely new. I’ll look into it further online. If you ever find a link to a report of a similar issue, I would be happy to see!

Edit: I found this link, the issue reported appears to be very similar to what I’m seeing here https://libreddit.tux.pizza/r/kde/comments/jhqbnz/kde_plasma_rendering_problem_black_squares/

Edit 2: I finally solved my problem! It was indeed an old problem already reported somewhere.

I agree with that. I suspect you might be right. SDDM (Breeze) is also weird with transparency. However, I just installed materia-kde but unfortunately the problem persisted (screenshot attached). Before that, I ran apt purge kde* plasma* libkf* and apt install kde-full. That too didn’t solve my problem.

Tasksel seems to be correctly set,

~$ tasksel --list-tasks
i desktop       Debian desktop environment
u gnome-desktop GNOME
u xfce-desktop  Xfce
u gnome-flashback-desktop       GNOME Flashback
i kde-desktop   KDE Plasma
u cinnamon-desktop      Cinnamon
u mate-desktop  MATE
u lxde-desktop  LXDE
u lxqt-desktop  LXQt
u web-server    web server
u ssh-server    SSH server
i laptop        laptop

I tried apt reinstall kde-full, but unfortunately nothing changed.

I audibly laughed at this. I actually knew I was entering treacherous waters by running apt full-upgrade in Sid, but still thought “well, we’ll see…”

Thanks! Some packages were installed, but it didn’t solve it yet (even after a reboot).

...
Install: orca:amd64 (46.0-1, automatic), libpcaudio0:amd64 (1.2-2+b2, automatic), speech-dispatcher-audio-plugins:amd64 (0.11.5-4, automatic), python3-brlapi:amd64 (6.6-5, automatic), xbrlapi:amd64 (6.6-5, automatic), speech-dispatcher-espeak-ng:amd64 (0.11.5-4, automatic), libsonic0:amd64 (0.2.0-13, automatic), sound-icons:amd64 (0.1-8, automatic), python3-speechd:amd64 (0.11.5-4, automatic), libespeak-ng1:amd64 (1.51+dfsg-12, automatic), python3-louis:amd64 (3.29.0-1, automatic), x11-session-utils:amd64 (7.7+6+b1, automatic), xkbset:amd64 (0.8-1, automatic), task-desktop:amd64 (3.75, automatic), libdotconf0:amd64 (1.3-0.3+b1, automatic), xorg:amd64 (1:7.7+23, automatic), perl-tk:amd64 (1:804.036+dfsg1-2+b1, automatic), x11-apps:amd64 (7.7+11+b1, automatic), speech-dispatcher:amd64 (0.11.5-4, automatic), espeak-ng-data:amd64 (1.51+dfsg-12, automatic), task-kde-desktop:amd64 (3.75), xinit:amd64 (1.4.2-1, automatic)
End-Date: 2024-03-26  17:42:41

If I am not mistaken, I used a Debian KDE live image from the official repository then switched the mirrors from Bookworm to Sid. The system went months without a single issue, then this happen.

Your suggestion will actually be my solution of choice if everything else fails: reinstall / and import relevant files from a backup that I already have.

Thanks for the tip! However, I tried apt reinstall kde-full and apt --fix-broken install, but no packages were installed and (unsurprisingly) the problem still persists.

The Gentoo sub had such a nice subtitle because it is indeed accurate! Definitely one of the things I like the most about “flexible” distros.

“Gentoo Linux: Because you like it when the power is in your hands”

Yeah, I tried installing kde-full but it didn’t solve my problem.

Thanks for the recommendation! I will definitely do it when I eventually install some other distro in the future.

I do store regular backups of this machine, but not of /var. I can always reinstall Debian (or whatever other distro), while keeping other relevant configs intact (stored in the backups) and not lose any critical data.

I commented below that I did check /var/log/dpkg.log, but it didn’t help much due to the high number of packages removed that day.

At this point I am more curious to learn more about KDE and what is causing the problem, since other desktop environments (I installed mate) seem to work fine.

Sadly I am not using BTRFS for my root directory on this specific system. If I end up deciding to reinstall, I will definitely go back to BTRFS to avoid such problems.

Debian actually has a KDE group named kde-full. I reinstalled it but the issue persists, which was honestly surprising to me.

~$ sudo apt install kde-full
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
kde-full is already the newest version (5:147).
0 upgraded, 0 newly installed, 0 to remove and 87 not upgraded.

The new user idea was really clever, thanks for the suggestion! I will try that now and see.