bug#45944: 28.0.50; Mailabbrev sometimes hangs

[Katsumi Yamaoka <yamaoka@jpl.org>]

Hi,

When expanding a mail abbrev in the message header of a message-
mode buffer, it sometime hangs like it goes into an infinite loop.
It's ok for Emacs on the Linux platform because I can break it by
C-g.  However, on Cygwin, only I can do is to kill Emacs, as C-g
doesn't break a Lisp infinite loop on Emacs built on that platform.

Though I haven't found out the cause of the problem so far, it seems
to happen when there are some kinds of contents in the message body,
and seems not to happen if the body is empty.  So, a workaround I'm
using now is as follows:

(defadvice abbrev--default-expand (around my-narrow-to-head activate)
  "Narrow the messge buffer to the headers when expanding an abbrev."
  (save-restriction
    (when (message-point-in-header-p)
      (let ((cur (point)))
        (message-narrow-to-headers)
        (goto-char cur)))
    ad-do-it))

Thanks.