I bet you could compile an old Xlib (or Xt/Motif) application on Linux, but good luck running a binary from way back when -- the libc/glibc and a.out/ELF switch should put breaks on that.
On the other hand, if your actual premise is open source, there's simply less incentive of avoiding breaking binary compatibility. I bet there were a few angry words and stomped feet in the last decades at Microsoft because of having to maintain that.
Not that glib/gtk are poster childs for even source-level compatibility, not even within one major version. There's a reason why several projects are switching to Qt.
(I'm doing my Cato bit here: I still think that they should've done an Xt toolkit instead of inventing the wheel yet again. There certainly was some experience there, given that GIMP 0.x was written for Motif.)
On the other hand, if your actual premise is open source, there's simply less incentive of avoiding breaking binary compatibility. I bet there were a few angry words and stomped feet in the last decades at Microsoft because of having to maintain that.
Not that glib/gtk are poster childs for even source-level compatibility, not even within one major version. There's a reason why several projects are switching to Qt.
(I'm doing my Cato bit here: I still think that they should've done an Xt toolkit instead of inventing the wheel yet again. There certainly was some experience there, given that GIMP 0.x was written for Motif.)