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

[Eli Zaretskii]

> Date: Sat, 13 Aug 2022 22:12:42 +0300
> Cc: 56682@debbugs.gnu.org, Eli Zaretskii <eliz@gnu.org>,
>  Stefan Monnier <monnier@iro.umontreal.ca>
> From: Dmitry Gutov <dgutov@yandex.ru>
> 
> > Moreover it does not offer any 
> > protection against modes which use widen in their font locking 
> > routines.  Which was the main reason to add the locked narrowing feature.
> 
> The only modes we know that do that (CC Mode)

AFAIU, currently _all_ modes widen, because font-lock does.  That part
was not yet fixed in font-0lock, was it?

> are also incompatible with your code. Your "protection" breaks it.

For some value of "breaks".

And that is a deliberate "breakage": we consider responsive Emacs more
important than correct fontification or "accurate" operation of modes
such as CC Mode.  As was already explained numerous times here, and
arguing about that should by now be understood as being futile.
People who disagree with that default can customize
long-line-threshold to a larger value or to a nil value.