bug#56682: Fix the long lines font locking related slowdowns

[Eli Zaretskii]

> From: Ihor Radchenko <yantar92@gmail.com>
> Cc: Stefan Monnier <monnier@iro.umontreal.ca>,  56682@debbugs.gnu.org,
>   Gregory Heytings <gregory@heytings.org>,  Eli Zaretskii <eliz@gnu.org>
> Date: Mon, 15 Aug 2022 20:08:12 +0800
> 
> However, I do insist on my statement that expecting major-mode to not
> widen is a bug.

We only don't expect that when the mode is invoked from redisplay.