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

[Eli Zaretskii]

> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Cc: Eli Zaretskii <eliz@gnu.org>,  56682@debbugs.gnu.org,
>   monnier@iro.umontreal.ca
> Date: Tue, 26 Jul 2022 08:51:54 +0200
> 
> Gregory Heytings <gregory@heytings.org> writes:
> 
> > Hmmm...  After 350e97d78e, Isearch locks Emacs with
> > toggle-truncate-lines. Recipe:
> >
> > C-x C-f long-line.xml
> > C-x x t
> > C-s </
> >
> > You have to kill Emacs, C-g does not work.
> 
> That's kind of funny :-):
> 
> (setq isearch-lazy-highlight nil)

Did you try this on master or on the feature branch?

And I found that customizing lazy-highlight-max-at-a-time and
lazy-highlight-interval can alleviate the problem to some extent.

The reason seems to be that lazy-highlighting tries to highlight every
match "in th window", but its interpretation of "in the window" seems
to be "buffer position before window-end position".  Which of course
includes a lot of stuff in a window with truncate-lines.