Emacs qualifies as a window manager in my book! ;)
That code you linked to is from Mitch Bradley's "Forthmacs", which ran on Sun workstations including 68k i86 and SPARC, and also Atari ST, Mac and other systems. He developed it into the "Open Boot ROM" architecture, which was used in Sun workstations and Apple PowerPC Macs as well as the OLPC children's laptop.
https://github.com/ForthHub/ForthFreak/blob/master/Forthmacs
https://en.wikipedia.org/wiki/Open_Firmware
http://wiki.laptop.org/go/Open_Firmware
On SunOS, Forthmacs had a library clink.f with the ability to dynamically relocate and link Unix libraries so that you could call them from Forthmacs, pass arguments on the stack, etc. SunOS didn't actually support shared libraries or dynamic address relocating at that time, so Forthmacs simply ran the Unix linker utility to create a file with the library relocated to the desired address space in the FORTH dictionary, and then read that file into memory, define its symbols in the FORTH dictionary, and let you access its variables and call its functions directly from FORTH!
That's how Mitch originally integrated MicroEmacs with Forth to make Forthmacs, and how I later integrated "uwm" into FORTH: I refactored uwm so instead of having an event loop in the main function, it was a library that could be called by FORTH, which would link the library in and run the main loop itself, calling into the library as needed to initialize and handle specific events (_uwm_init, _uwm_poop).
http://www.donhopkins.com/home/archive/piemenu/uwm1/fuwm-mai...
Here's the glue that links in the uwm library from fuwm.out:
http://www.donhopkins.com/home/archive/piemenu/uwm1/load-fuw...
.( Loading...) cr
requires tasking.f
requires uwm.f
requires clink.f
.( Linking...) cr
"" fuwm.out clink
.( Linked!) cr
My head is spinning. Amazing history part still.