bug#56393: Actually fix the long lines display bug

[Eli Zaretskii]

> Date: Thu, 21 Jul 2022 07:49:44 +0000
> From: Gregory Heytings <gregory@heytings.org>
> cc: gerd.moellmann@gmail.com, larsi@gnus.org, 56393@debbugs.gnu.org
> 
> > As far as I can see, font locking is the one and only remaining cause of 
> > slowdowns that is under control of Emacs.  Mode-specific slowdowns can 
> > of course not be solved in Emacs.  So what about simply turning font 
> > locking off unconditionally when long lines are detected in a buffer, 
> > until the font locking slowdowns are fixed?
> >
> 
> (Doing that would also make it easier to get feedback about other causes 
> of slowdown, if any.)

That could be so, but I'd prefer first to remove the elephant in the
room.  It's not like people edit such problematic files all the time,
so feedback will probably come in quickly only from people who
deliberately do that because they want to test this feature.