Nope this is not the issue. It is an issue with the themes. Here is the file ~/.config/gtk-3.0/gtk.css
@import 'colors.css';
@import 'window_decorations.css';
The file window_decorations.css doesn’t exist.
This is a default KDE installation.
Here already on the forum: GTK themes doesn't work correctly after Plasma 5.20 upgrade (Manjaro testing) - #6 by michaldybczak
Didn’t find a bug report.
//EDIT: switching the GTK theme from Adwaita-dark to Adwaita removes this line. switching back to Adwaita-dark doesn’t add the line back, so it must have been an issue now fixed by re-applying the GTK theme.
//EDIT2: and after reboot the line is back so no the issue is not fixed.