Hi there, welcome to the forum.
Open KDE settings, search / go to “Window decorations” and check the window borders drop-down list on top left.
Please always include the output of your garuda-inxi, as required by the topic template.
Edit: I see, they remain after changing to none. Let’s wait for someone who knows better
System Settings → Window Management → Desktop Effects - > Rounded Corners.
Either deactivate the “Rounded Corners” effect or configure the color manually in the “Outlines” tab in the settings for this effect.
I cant say ive had that and ive been using it since it was re written for kde 6. though im no longer using my nvida cards so i haven’t tested there.
*Edit i did test it an i did see before i restarted white line going across the blurred area but even removing all effects they were still there so i rebooted since i put my system to sleep lat night an had not restarted it yet. after that it worked fine. Note i only saw the effect when GPU screen recorder was recording. The odd thing is it didn’t come over in recording. maybe kwin or mesa? just a rough guess.
After reboot the others all look the same. Sorry for the quality its webp,
At least for me between the two days there have been no update changes
--- Version 156:
<<< /nix/store/5nvklcvhya6mppgk46jr74hrzzxzfk53-nixos-system-nixos-25.05.20250309.e3e32b6
>>> /nix/store/bq0zfyqh82hnac2rycvzwajdzbf4g9sh-nixos-system-nixos-25.05.20250309.e3e32b6
No version or selection state changes.
Closure size: 3842 -> 3842 (5 paths added, 5 paths removed, delta +0, disk usage -344B).
--- Version 157:
<<< /nix/store/bq0zfyqh82hnac2rycvzwajdzbf4g9sh-nixos-system-nixos-25.05.20250309.e3e32b6
>>> /nix/store/qwhaf2m40kn1fvklq158h1n0k4cmm8sg-nixos-system-nixos-25.05.20250309.e3e32b6
No version or selection state changes.
Closure size: 3842 -> 3842 (4 paths added, 4 paths removed, delta +0, disk usage +3.0KiB).