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

[Eli Zaretskii]

> Cc: 56682@debbugs.gnu.org
> Date: Fri, 22 Jul 2022 17:51:33 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> 
> > Date: Fri, 22 Jul 2022 10:16:52 +0000
> > From: Gregory Heytings <gregory@heytings.org>
> > cc: Eli Zaretskii <eliz@gnu.org>
> > 
> > 
> > on my laptop, with Emacs from master from a week ago, take respectively 
> > 150 seconds and 40 seconds.  With the improvements on master, 1 is 
> > instantaneous but 2 still takes about 5 seconds.  Now, with the changes in 
> > the feature/long-lines-and-font-locking branch, both are instantaneous. 
> > The price of that speedup is that some portions of the buffer will be 
> > mis-highlighted, which is unavoidable.
> 
> The assertion below can now be violated:
> 
>       if (it->narrowed_begv)
>       {
>         record_unwind_protect (unwind_narrowed_begv, Fpoint_min ());
>         record_unwind_protect (unwind_narrowed_zv, Fpoint_max ());
>         SET_BUF_BEGV (current_buffer, it->narrowed_begv);
>         SET_BUF_ZV (current_buffer, it->narrowed_zv);
>         specbind (Qinhibit_widen, Qt);
>       }
> 
>       val = Vfontification_functions;
>       specbind (Qfontification_functions, Qnil);
> 
>       eassert (it->end_charpos == ZV);  <<<<<<<<<<<<<<<<<
> 
> because of the "narrowing".  (I actually saw this assertion violation
> once on the branch, but I cannot reproduce it.)

Sorry, it's very easy to reproduce: visit dictionary.josn, and then
type C-v several times until it happens.