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

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Eli Zaretskii <eliz@gnu.org>,  56682@debbugs.gnu.org
> Date: Sun, 31 Jul 2022 03:25:46 -0400
> 
> How 'bout we do the reverse then: set the narrowing, but let-bind
> a variable to indicate that we're inside a line-length-induced
> narrowing, together with the previous narrowing bounds, so jit-lock or
> its clients can undo the narrowing when needed?

I'd rather prefer an extension of 'widen' that would be able to undo
the locked narrowing "when needed".  That way, jit-lock clients should
actually do something to request that, instead of letting them keep
the current code that widens whenever they feel like it.