Missing client side decorations support

I tested the desktop options in Debian's installer (using Debian testing). Lomiri appears to be unique in not supporting client side decorations. Even Unity (which isn't a Debian desktop but is an Ubuntu desktop) supports them.

Test Case

1. Make sure you don't have gtk-nocsd or libgtk-nocsd0 installed.
2. Install simple-scan.
   • If you're using Debian or Ubuntu, you can take care of all of that, by running sudo apt install simple-scan libgtk-nocsd0
   • Reboot after making those changes.
3. Run simple-scan (shows in the launcher as Document Scanner). It's a gtk4+libadwaita app.

Lomiri shows the server side decorations even though the app uses client side decorations and has a close button of its own.

Unity

Lomiri