Problem with Window decorations - not properly rounded [6.3.5]

[KarimElsayad247]

If you look closely, you'll see that corner isn't exactly rounded, and leaves some sort of background.

Problem happens on both X11 and Wayland.

System details:

Operating System: openSUSE Tumbleweed 20250509
KDE Plasma Version: 6.3.5
KDE Frameworks Version: 6.13.0
Qt Version: 6.9.0
Kernel Version: 6.14.5-1-default (64-bit)
Graphics Platform: X11

[KarimElsayad247]

Problem also happens with Breeze decorations, so I don't think it Catppuccin related

[KarimElsayad247]

Problem disappeared after I changed shadow size to "None"

so it seems the problem is related to shadows in general.

[KarimElsayad247]

Actually, when I add the shadow again, the problem remains fixed.

The problem surfaces again when I change size of window decoration buttons.

So a workaround whenever it happens is to switch to "Breeze" decorations and turn shadows off an on again.

[obipoketbobo]

this problem is due to how the Blur KWin effect works; it doesn't take rounded corners into account. use this fork of the blur effect and set the rounded corners to match, i found setting "Window top corner radius" to 20 and Anti-aliasing to 3 worked great for me