bug#30364: 26.0.91; thread crash on macos

[Eli Zaretskii]

> From: Aaron Jensen <aaronjensen@gmail.com>
> Date: Tue, 27 Feb 2018 21:33:01 -0800
> Cc: Eli Zaretskii <eliz@gnu.org>, Andreas Schwab <schwab@linux-m68k.org>, 
>       Noam Postavsky <npostavs@gmail.com>, 30364@debbugs.gnu.org, 
>       Noam Postavsky <npostavs@users.sourceforge.net>, Alan Third 
> <alan@idiocy.org>
> 
> On Tue, Feb 27, 2018 at 9:30 AM, Paul Eggert <eggert@cs.ucla.edu> wrote:
> > Nobody else commented, so I reformatted the patch a bit to be more in the
> > Emacs style and installed the attached into master.
> 
> I forgot, but given that this is a show-stopper bug for a new
> headline/experimental feature for Emacs 26, would it be worth applying
> this to the emacs-26 branch as well?

What feature is that, and why is this a showstopper?