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

[Dmitry Gutov]

On 05.08.2022 16:41, Gregory Heytings wrote:
> > > What I'm not sure of is how useful is a "font-lock with arbitrary 
> > > narrowing", where portions will be highlighted as strings rather than 
> > > code (and vice-versa).  I don't have enough experience with it yet to 
> > > be sure.  Taking a step back, I suspect that the only "real" solution 
> > > is something like `jit-lock-defer` coupled with a way to perform the 
> > > font-lock (and syntax-ppss/propertize) in the background.
> >
> > IIRC I heard that "some other editors" take the approach of 
> > restricting syntax-highlighting to just the beginning of a large file.
>
> Other editors just give up syntax highlighting altogether even with the 
> 18 MB file (and you cannot edit really large files with them).

In my testing, with my patch, the 18 MB file works reasonably well and 
has good syntax highlighting.

> But if you're so annoyed by mis-fontification, why don't you just turn 
> font-lock mode off?

"If you're annoyed by Emacs's performance with large files, why don't 
you just never open them?"

I like font-lock and the visual cues that come with it. Only 
font-locking the first 1 MB of a large file seems like a good 
compromise: show correct highlighting where we can with reasonable 
performance, and omit it in the rest of the file.

> Also, why did you not protest vehemently when Stefan added 
> syntax-wholeline-max, which also causes occasional mis-fontification?

I have replied to this exact question in an earlier email. We can 
continue this line of inquiry in that subthread.