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

[Stefan Monnier]

>>> That's true, but with such big files, the initial scan is slow.  So the
>>> scenario is simple: you open a big enough file, type M->, and C-p. M->
>>> will be instantaneous, and C-p will take a while, because of syntax-ppss.
>> Really?  I'd expect that `M->` is slow because of `syntax-ppss` (called by
>> font-lock) and then `C-p` is instantaneous.
> Yes, really.  M-> is fast because syntax-ppss is called inside
> fontification-functions, which are evaluated in a small portion of the
> buffer (with locked narrowing).

Ah, you're talking about a file with a long line.  I was thinking of just
a normal large file.

> And C-p is slow because post-command-hook is (or rather was) not
> subjected to the same locked narrowing.

I wonder what `C-p` has to do with `post-command-hook`.
After all, that same `post-command-hook` is also run after `M->` as well.


        Stefan