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

[Eli Zaretskii]

> Date: Wed, 3 Aug 2022 03:26:02 +0300
> From: Dmitry Gutov <dgutov@yandex.ru>
> Cc: 56682@debbugs.gnu.org, monnier@iro.umontreal.ca
> 
> On 03.08.2022 03:00, Dmitry Gutov wrote:
> > On 02.08.2022 19:19, Gregory Heytings wrote:
> >>
> >>>> Try to open the dictionary.json with Emacs on master a month ago.
> >>>
> >>> I believe it can also be done with the current master, just after 
> >>> setting long-line-threshold to the nil value.  Right?
> >>>
> >>
> >> Indeed.  With master from one month ago it's even more crystal-clear 
> >> that you see the statu quo ante.
> > 
> > If I set long-line-threshold to nil, does that also disable the 
> > redisplay optimizations related to long lines?
> > 
> > Ones that caused scrolling delays even after the buffer has been fully 
> > fontified.
> 
> I mean those that *fixed* the said scrolling delays, of course.

I don't think I understand which changes you had in mind (could you
give a less vague pointer?), but I think the answer is NO: that
variable disables only the recent changes related to long lines.