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

[Dmitry Gutov]

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.