defaults write org.gnu.Emacs NSAutoFillHeuristicControllerEnabled -bool false
I've been using this for a couple days now. FWIW
- With GNU Emacs, this seems to work just fine. Hurray!
- With Emacs-Mac port, which many people like (including myself): I haven't had any luck so far. In particular, if you (like me) like to compile your own from source, doing this on macOS 26 (with XC26 etc) will compile but the resulting binary will very quickly (in seconds to minutes) become unresponsive. If you use a precompiled binary (including Homebrew cask) compiled on an earlier macOS version, that may be fine (provided you set NSAutoFillHeuristicControllerEnabled as above).
Somehow the combination of the compiler & library (runtime & statically linked) is breaking Emacs-Mac for me. I haven't quite figured out why, and probably don't have time right now to get to the bottom of this...