I had seen this before and I don't know why I didn't pay more attention, but it certainly presents a perfect and normal GTK 3 GUI on my ubuntu 23.04
From the same code on mac and windows, it also generates their respective native UI, cocoa for mac I suppose.
haha. No idea if either of the other two still work, it's been some time. There may be some improvements I can make, I'm familiar enough with programming GTK from the olden days of the early gtk3 releases, almost got used to working with glade.
But both mac and windows have supported apps going all the way back to before NT, so the old 32 bit UI libraries also work, so basically, probably, they will work, just as they did when it was last updated.
On windows 11, linux native apps can anyway run directly as single app Weston instances. They didn't even fix the funny original unthemed weston window frames. They look almost identical to windows 7 Aero windows without transparency and blur, same rectangular control buttons.
I hope they do work though, cos that would be neat. I was dreading dealing with some half done thing as is the norm with Go GUIs, and I expect I may end up adding features, will just most likely be some C shims and bindings to get things that were missing.
I dunno about windows, but I know CSD has been a thing on it since windows 10 in a big way. Headerbars are a nice idea and I honestly doubt that GTK4 really needed to be a breaking change, it's ridiculous having 3 versions of the same UI toolkit in use on one X session.