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

[Dmitry Gutov]

On 01.08.2022 01:50, Gregory Heytings wrote:
> > But one big slow scan (and how slow it is actually depends on a 
> > particular major mode) followed by responsive editing sounds much 
> > better than what we've had before.
>
> Indeed.  But then the question is: is it possible to do that scan while 
> opening the file, before it becomes editable?

IIUC this state of affairs is caused by your chosen approach to speeding 
up font-lock (hard narrowing while it is called), which makes the 
initial call to syntax-ppss happen inside that narrowing as well.

The alternative being that font-lock would call syntax-ppss right away 
with no restriction, but then only apply highlighting to limited parts 
of the buffer.

> It is way better to wait
> a few seconds more while the file is being opened than to wait before
> two basic motion commands when the file is already opened.

I agree, yes.

> > So I would recommend against trying to solve this part right now.
>
> It doesn't only solve the syntax-ppss problem, it also makes 
> flyspell-mode usable in such files, for example.

Does flyspell-mode always scan the full buffer?