Make macvim set to do non-native fullscreen (NNFS) mode. Then do mvim -u NONE -U NONE -N then do:

:set co=80
:set fuopt-=maxhorz
:set fu

What should happen is that MacVim enters NNFS, but &co is still 80. What actually happens is that MacVim acts as if the -=maxhorz didn't happen.

It might be related to one of the additional difficulties from #488.