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

[Eli Zaretskii]

> Date: Mon, 25 Jul 2022 21:47:55 +0000
> From: Gregory Heytings <gregory@heytings.org>
> cc: Gerd Möllmann <gerd.moellmann@gmail.com>, 
>     56682@debbugs.gnu.org, monnier@iro.umontreal.ca
> 
> > And I've found a serious sink of CPU cycles under truncate-lines, and 
> > installed a fix on the feature branch.  Gerd, if you have time to 
> > eyeball the fix and comment on it, I'd appreciate.  It's commit 350e97d 
> > on the branch.  (I can post a more detailed explanation of what I did 
> > and why, if that would help, because the code and the functions it calls 
> > are somewhat tricky.)
> 
> 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.

I see the same on master, where that change is not yet installed.