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

[Dmitry Gutov]

On 05.08.2022 15:16, Gregory Heytings wrote:
> > > If and when it is solved, we could revisit this issue.
> >
> > The locked narrowing currently in effect makes it impossible to 
> > investigate this problem or improve the behavior :-(
>
> How so?  It is fully backward compatible, just (setq long-line-threshold 
> nil).  If what you want is only to remove the locked narrowing, comment 
> out the relevant line in xdisp.c.

That brings back the performance problems in redisplay, which have a 
more pronounced effect, overshadowing both issues and potential 
improvements in font-lock.