I’m using Plasma 5.27.12 from Kubuntu backports in a machine that runs Ubuntu 24.04.2 LTS (upgraded from 22.04 LTS). Yesterday I fixed an issue with AMD GPU drivers and, since then, all GTK applications (I think that’s the common pattern) feature a thick border decoration on non-maximized windows.
Microsoft Edge:
GNOME System Monitor:
It’s particularly annoying because I use multiple monitors and when I stick windows the border extends to next display.
Is there a way to remove it?
OMG, more things broke than I had initially noticed. KDE apps no longer cast shadows, even though themes defines them 
I’ve discovered that when I use the keyboard short-cut to enable/disable compositor, this gets logged in /var/log/syslog:
kwin_core: Compositing disabled: no OpenGL support
kwin_core: Compositing is not possible
Check your drivers !
Perhaps you need a reboot.
Yes, but what exactly needs to be fixed? My laptop has hybrid graphics and I’m using open source AMDGPU and proprietary NVIDIA, I don’t even know which ones are to blame.
(I’ve rebooted several times already.)
Did you try a wayland session ?
It might not run at all, but since it manages the GPU more directly then, it might have a better behavior, not sure.
Ubuntu 24.04.2 LTS
That likely means a 1 year old driver stack and kernel.
That won’t help.
Also 5.27 is maintained only for security or catastrophic issues.
Still your system should be able to run with Gpu acceleration.
Stupid company policies. If it was for me, I’d be using Neon or Fedora.
My system supported GPU acceleration until yesterday
BTW, I think Wayland is not an option in my distro, not at least for Plasma.
For Kubuntu it has been an option for years, need to install a specific package to enable it.
Ah… I guess it’s plasma-workspace-wayland and maybe plasma-wayland-protocols. I can give it a try.
I think it is plasma-wayland-session or something along those lines.
Installing plasma-workspace-wayland and plasma-wayland-protocols enabled Wayland entry in display manager and now my shadows are back.
From my little knowledge about the X11 vs Wayland controversy, I know some applications are not ready (I’ll check Teams screen sharing later) and, indeed, I cannot drag KNotes windows from title bar, neither regular drag nor Alt+Drag (interestingly, Win*+Drag works!).
I’ll figure out what’s better or at least less worse.
(*) Sorry, I never remember how this key is called on Linux.
*Sigh*
And in plasma 5.27, wayland was mostly usable already, but a lot has been improved since.
Since drivers appear to be alright, as Wayland demonstrate, I’m positively sure that some package or setting was gone when I was fixing stuff. If only I knew which one…
Yes, there is a problem with your drivers. You have no more compositing enabled, hence no shadows for kde apps and the thick borders around gtk apps. The thick borders are the gtk shadows, their transparent part is now opaque. Fix your drivers, then your problems will disappear. Don’t have high hopes on wayland, kde/plasma 5 is not very usable with it, wayland is only stable in the plasma 6 series.
Disable all backport repositories, remove all kde packages and install them again from the standard Kubuntu repositories. The backports come without warranty and are not officially supported. Then fix your drivers, if you have hybrid graphics, a mismatch between the drivers is most likely the culprit. Remove all your nvidia drivers and remove your xorg.conf and related configuration files. Probably your proprietary nvidia driver messed up your configuration. Once you have compositing running again with your amd drivers, add back your nvidia driver, see if you have problems, if so, try to find the reason exactly, then add back your backports. Do everything bit by bit, so you can triage the package or configuration that causes your graphics to break.
I doubt @kAlvaro is using Nvidia drivers with an AMD GPU. (Though that would certainly cause problems!)
I see from later comments that I am wrong! My mistake.
To put things into perspective:
On Zenned we still consider the Wayland session experimental, and you are warned about it on the login screen itself.
We use the latest open source Nvidia drivers, along with Optimus Manager.
I followed your advice and the problem is fixed now.
I can’t tell the exact package that was causing it, because I tried lots of things that didn’t work. But, at some point, I tried to purge several mesa and libmesa packages in order to reinstall them later, I let apt cascade-remove the packages which depended on them (essentially, everything related to desktop environments), and when I reinstalled everything back my shadows returned.