Thinking aloud: If we really, really wanted to get this working, the desktop process would need to create and manage wl_surfaces for all the windows (handling both input and output). The other processes would have to render off-screen and then send the buffers to the desktop process for presentation.
This is pretty much what I expect we would need to achieve at some point, if we want things like DirectComposition, and cross process rendering like applications can do on Windows. Though ideally we would want to keep a fast and direct path for the most common case where applications are simply drawing their surfaces.
That would also depend on the backend capabilities, and I think it's not much of an issue with X11 as you can just create children surfaces of another process window if you like. Then if Wayland doesn't allow that it would have to use the indirect path all the time.
The desktop process would also need to forward Wayland events to the other processes for handling. We would be trying to meld the multiple processes into one.
I'm not sure to understand that very well, the idea is more to move the boundaries of the Windows world a little bit further and have a dedicated compositor which would be the interface with the host instead of every single process. I'm not sure why host events would have to be forwarded at all with this architecture.
So, I guess me saying "is not possible" was not entirely accurate, but I think the complexity of such a solution is extreme enough to effectively make it "not possible". But perhaps there is workable approach that escapes me?
Well... like I said, this is a long road. Doesn't look much more impossible than what we usually do :sweat_smile:.