Got gtk4 client-side window decoration size details dynamically now - https://github.com/falkTX/wayland-audio-plugin-test/blob/51b301ef46fc9d20d96716a3ce1c5589b66344cf/gtk4-host.c#L31-L62

No need for super hacky "render window offline and then fetch position based on pixels", instead it is just:
1. create dummy window
2. fetch its initial size (200x200 on my laptop)
3. add a title bar
4. get title bar size and updated window size
5. x,y offsets are (new size - old size) / 2

picture shows this working nicely, I have an intentional 20px padding around the yellow rectangle