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

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Gregory Heytings <gregory@heytings.org>,  dgutov@yandex.ru,
>   56682@debbugs.gnu.org
> Date: Sat, 13 Aug 2022 09:37:06 -0400
> 
> > Is 20MB single-line JSON file fairly representative of such files out
> > there?  Do we know of JSON file with much longer lines, for example
> > 50MB or 100MB?
> 
> IIUC it's very common for JSON files to be made of a single line, so the
> line length is the file's length and since those files are
> machine-generated I don't expect 100MB to be much less common than 20MB.

I think I'd prefer some actual data points.  It's clear that larger
JSON files can happen, the question is how frequently they do happen.

> > If 20MB is reasonably representative of the long-line JSON, then
> > perhaps at least json-mode could consider resetting
> > long-line-threshold to nil.
> 
> IIUC a nil value for `long-line-threshold` makes Emacs completely
> unusable with such long lines even if you disable all the bells
> and whistles.  Dmitry's suggestion is to skip the locked-narrowing in
> some cases, but not to set `long-line-threshold` to nil.

That's what I actually meant.

However, if much larger JSON files exist, we should first see what
that proposal produces with such larger files.