Constantine Vetoshev <vetoshev@gmail.com> writes:

> 5. When I run Emacs 26.1 under lldb, everything works! No crashes, no
> error messages.

Ugh, one of those.

> After copying term.el from 25.3 into 26.1 (and deleting term.elc),
> 26.1 still exhibits the bug.
>
> Something must have changed in the macOS-specific pieces of Emacs
> between 25.3 and 26.1 which started causing this crash.

Okay, I'm going to guess it's the change to use vfork.  Here's a patch
which should undo that change (I can't test it on macOS, and it's just
assembled from git log --grep=vfork, so I may have missed something).
Try it out and see if it avoids the crash.