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

[Eli Zaretskii]

> Date: Sat, 6 Aug 2022 13:50:41 +0300
> Cc: 56682@debbugs.gnu.org, gregory@heytings.org, monnier@iro.umontreal.ca
> From: Dmitry Gutov <dgutov@yandex.ru>
> 
> >> I'm not seeing any particular sluggishness in these operations when
> >> visiting dictionary.json.
> > 
> > Numbers, please.  You have a very fast machine, so what doesn't look
> > sluggish on your system could very well be so on others.
> 
> How do you measure these operations including the redisplay lag?

By timing them.  With benchmark-run or similar, if it isn't slow
enough to be measured "by hand".

> > Linear scaling is less optimal than O(0), which is what the current
> > solution produces.
> 
> Correct syntax highlighting requires parsing the buffer from the 
> beginning. Otherwise we get random results, essentially.
> 
> Just like depicted on my latest screenshot for downloadify.js: the 
> beginning of the narrowed region ended up inside a string, and as a 
> result several screenfuls were entirely mis-fontified, with strings and 
> non-strings inverted.

Yes, and IMNSHO responsiveness is more important than correctness in
these cases.