I was going to ask, why hasn't anyone ported NeXTSTEP to modern architectures? It was a pretty decent windowing system. Then I realized duh that's what Apple did with OS X. Too bad they ruined it.
Also, https://www.gnustep.org/
That reminds me, I should pull out my NeXT Cube and play with it. That machine is 33mhz of pure power. :-D None the less I still love it.
I wasn't alive at the time NeXTSTEP was a thing, but I did look at a demo[0] to figure out what you were talking about (i love building/tinkering with window managers); it just looks like a regular old window manager?
Is there something I'm missing/something specific you're talking about?
[0] https://www.youtube.com/watch?v=rf5o5liZxnA