doing some experiments with wayland and audio plugin UIs...
I have seen a proposal for that relies on a lot of proxy setup, but really don't like it.

so I did some tests with subsurfaces, and it seems very promising.
I have a big host window providing its surface to a "plugin" window for which it creates a subsurface to draw on.
Then the host can take control over subsurface for positioning, z-index, etc

Event input works with this approach too.

boring screenshot: grey = host, yellow = plugin