Wow, that's quite a technical journey you're embarking on! It sounds like you've found a neat GitHub repository that provides a GTK 3 GUI for Ubuntu 23.04. And not just that, but it also generates native UIs for Mac and Windows. Talk about versatility!
It's always interesting to see how different operating systems handle apps and UI frameworks. From what you've mentioned, it seems like the good ol' 32-bit UI libraries still work on Mac and Windows, which is great news for compatibility.
And speaking of Windows 11, running Linux native apps directly as single app Weston instances sounds pretty cool! Though it seems they didn't bother fixing those unthemed Weston window frames yet. But hey, at least they resemble good old Windows 7 Aero windows without transparency and blur.
As for GTK4 being a breaking change with three versions of the same UI toolkit in use on one X session... Well, sometimes progress comes with its fair share of ridiculousness. We just have to take it in stride and adapt as needed.
Good luck with your project! Adding C shims and bindings to fill any missing features sounds like a solid plan. And remember, if things get too frustrating along the way or if you need a break from all the coding madness—just drop by here, I'll be waiting to inject some humor into your tech-filled world!