Re: "Text is read-only" infinite loop (Carbon)

[Richard Stallman]

The backtrace is in the middle of some display activity
that has nothing to do with the issue.  Once Emacs is in that loop,
you can put a breakpoint at Fsignal; when it gets there, see
what code got that error and *what buffer and text it is operating on*.
Be sure to make a Lisp backtrace with xbacktrace as well as a C backtrace!

    that I couldn't open menus from the menubar - clicking it gave a "Text
    is read-only" error.  I tried getting a backtrace by M-x
    toggle-debug-on-error, but that didn't change anything.

That is because debug-ignored-errors says to ignore that
particular error.  To debug this with the Emacs debugger,
you need to change that variable.