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

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: yantar92@gmail.com,  57207@debbugs.gnu.org
> Date: Thu, 18 Aug 2022 16:14:54 +0200
> 
> Simplified recipe:
> 
> (progn
>   (pop-to-buffer "foo.txt")
>   (flyspell-mode)
>   (insert (propertize (make-string 20000 ?x) 'display "foo")
>         "\n\n"))

Still no "Narrow" on the mode line.

But it's clear that one needs pre-command-hook or post-command-hook to
reproduce.

So the question becomes: why doesn't the narrowing get unwound?
Perhaps because we should do it in safe_run_hooks_maybe_narrowed,
instead of inside narrow_to_region_internal?