bug#56393: Actually fix the long lines display bug

[Eli Zaretskii]

> Date: Mon, 18 Jul 2022 17:11:23 +0000
> From: Gregory Heytings <gregory@heytings.org>
> cc: gerd.moellmann@gmail.com, larsi@gnus.org, 56393@debbugs.gnu.org
> 
> > M-x toggle-truncate-lines RET
> 
> BTW, there is now a C-x x t binding for that.

Old habits die hard ;-)  Besides, what I really type is

  M-x tog TAB tr TAB RET

> > Now simple cursor motion commands that use redisplay optimizations are 
> > fast, but commands that cause more thorough redisplay are as slow as on 
> > master.  As a simple example, try just "M-x" and wait until the "M-x" 
> > prompt appears in the minibuffer -- here it takes much longer, basically 
> > as long as the version on master.
> >
> 
> Yes, it's a font locking issue.  Turn font-lock-mode off and the problem 
> is gone.  As I said, I'll look at that later.

So you are saying that font-lock becomes much more expensive when
lines are truncated?  Because the comparison is with the same file and
font-lock turned ON, but without line truncation.