bug#45898: 27.1; wedged in redisplay again

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: gerd.moellmann@gmail.com,  larsi@gnus.org,  psainty@orcon.net.nz,
>   Emacs-hacker2018@jovi.net,  45898@debbugs.gnu.org
> Date: Wed, 29 Jun 2022 17:07:45 -0400
> 
> >> It doesn't seem to make much difference on the `medium_line.json`
> >> example from Phil, tho :-(
> > In what case does it make much difference?
> 
> None that I can see :-(
> 
> > And how did you test whether it makes a difference?
> 
> I just did
> 
>     src/emacs -Q ~/tmp/medium_line.json
> 
> and then tried to move around in the buffer, search with Isearch, and
> insert a few chars.
> I don't notice any difference with or without the patch :-(

If you start moving with C-n or C-v, Emacs becomes unusable
approximately around the same buffer position with and without the
patch?  If so, I guess the expensive part is somewhere else.

I'd install this patch regardless, because it sounds like TRT not to
let syntactic fontifications run amok this way.  We do similar stuff
in bidi.c for similar reasons.

Thanks.