Kwin will decorate the window if and only if the client tells it to. With Qt < 5.12 this was injected by plasma-integration plugin which relies on those XDG_CURRENT_DESKTOP For Qt >= 5.12 (with latest plasma) it will work out the box without plasma-integration For GTK apps you will need the latest GTK release.
>It would get the default wayland window deco What do you mean by default wayland deco?
