Implemented lower window when middle click on titlebar in gtk/mutter

[Mildred Ki'Lya]

Hi,

For a long time I delayed using Wayland on my computer for about three 
reasons: inability to restart the gnome-shell (Alt-F2, r), slowness in 
gnome-shell, and inability to lower client-side decorated windows on 
middle click like on X11. The performance getting better and better (and 
with the realtime option enabled in mutter) I decided to scratch my itch 
and implement lowering in waylang for Gtk client-side decorations.

My work is implemented as an upgrade to the gtk-shell protocol:

- https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1221

- https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/1821

It works, but I do not like how it works and the fact that it's Gtk 
specific (although other toolkits could implement the gtk protocol). 
Also, I don't like that it's the client that should look up the window 
manager configuration to know that middle clicking means lowering the 
window. It's up to the compositor to map the middle click to lower and 
not the client.

So I'm asking it here, what would be the wayland solution for a 
cross-toolkit and cross-environment solution?

I suppose that the client can declare the title bar area, and the 
compositor can then handle clicks in this area. is there something like 
that already existing? Is there anything that has prevented to do this 
before?

Thank you,

Mildred