Because of GNOME's insistence on not implementing Server Side Decorations, you can't not depend on libadwaita. This is what I imagine pull in all of the GTK dependencies.
You can very much draw a border on a windows and a "close" button without any libraries.
Usually, I'd understand if you're lazy and can't be bothered and just pull in some dependency to do it for you, but if you're implementing a toolkit, this is the kind of thing that is SHOULD provide.
