bug#57207: 29.0.50; Fontification is slow after e7b5912b23 (Improvements to long lines handling)

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: 57207@debbugs.gnu.org,  yantar92@gmail.com
> Date: Fri, 19 Aug 2022 19:25:24 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > I types "s".  Does it matter what you type?
> 
> Nope.

Ah, I know what's causing this: flyspell calls redisplay (via sit-for)
inside its post-command-hook.

So I think this basically means we cannot narrow the buffer while
calling pre- and post-command-hooks, because any of them can trigger
redisplay, and that will show "Narrow" on the mode line.

(I also understand why I didn't see the problem on my system: it's
because the recipe signals an error here, and flyspell-mode isn't
completely enabled.)