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 remember when I first learned about GNUstep in 2004 when I was in high school. It's a shame GNUstep never took off; we could have had an ecosystem of applications that could run on both macOS and Linux using native GUIs.

With that said, the dream is not dead. There's a project named Gershwin (https://github.com/gershwin-desktop/gershwin-desktop), which is a Mac-like desktop environment built on top of GNUstep. Gershwin appears to be heavily inspired by Apple Rhapsody (https://en.wikipedia.org/wiki/Rhapsody_(operating_system)) with some modern touches.