https://sourceforge.net/projects/motif/files/
https://en.wikipedia.org/wiki/Motif_%28software%29
(in some alternate universe, motif was under the x11 license and you would have motif v13 instead of GTK.)
Now we live in a time where we allocate GBs of RAM to eye candy that functionally accomplishes nothing. Then we make the case to rewrite the eye candy in increasingly "safe" languages, requiring even more RAM.
Enhanced Motif Window Manager https://fastestcode.org/emwm.html
and the full-fledged CDE desktop that uses Motif also:
https://sourceforge.net/projects/cdesktopenv/ (note that you want to firewall this somehow as the default settings on the background process ttdb can be a security hole)
One of the most annoying things about it was that it did not address the endianness of the arguments to the library functions. So it worked fine on big endian platforms, but not so fine on little endian ones (such as Intel).
It would still work okay if you byte swapped the arguments in and out of the library functions, but it just seemed silly to need to do that, and it made it more difficult to write portable code.
https://en.wikipedia.org/wiki/Windows_NT_3.1
[edit]
So, I guess the history of 'windows NT' is lost on many. 'NT' started with version 3.1 as the MS/IBM breakup from the joint OS/2 venture happened. It was their first real push into 32 bit protected mode operating systems and supported really crazy cool things like 'multiple processors' and totally different architectures than x86, like DEC. Give the link a look.