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

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Dmitry Gutov <dgutov@yandex.ru>,  56682@debbugs.gnu.org,  Eli Zaretskii
>  <eliz@gnu.org>
> Date: Tue, 02 Aug 2022 18:26:51 -0400
> 
> I strongly disagree with the necessity to make re-widening
> technically impossible, I find it fundamentally incompatible with
> Emacs's philosophy and can't see any practical justification here.
> Just narrow and make sure jit-lock.el and font-lock.el don't
> accidentally widen it.

What do you mean by "accidentally" in this context?  A program rarely
does anything "accidentally", it almost always does what the
programmer wanted it to do.