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

[Eli Zaretskii <eliz@gnu.org>]

> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Cc: Gregory Heytings <gregory@heytings.org>,  56682@debbugs.gnu.org,
>   monnier@iro.umontreal.ca
> Date: Tue, 26 Jul 2022 15:08:18 +0200
> 
> What about the following idea, in general.  Sorry if that is in the
> wrong sub-thread, I'm beginning to loose orientation a bit.
> 
> While iterating,
> 
> - remmember where we started, say it's 'start_pos'.
> 
> - when reaching start_pos + X, without reaching a line end, refuse to do
>   any complex stuff, no faces, invisible text and so on.
> 
> I could think of numerous variations of that theme.
> 
> I just loaded long-lines.xml in VSCode, and it seems to do something
> like that.  It says that "tokenization has been disabled" in the long
> line.  The long line doesn't have the highlighting that the shorter ones
> have, and VSCode also wraps the long line instead of truncating it.

We keep this possibility in mind all the time, you can see it in, for
example, the discussion whether to turn off line truncation.  But, as
I explained a minute ago, I think we haven't yet exhausted less
drastic measures, and there could still be opportunities for speedup
in the basic display code or thereabouts.  Our display is different
from that of VSCode, so the trade-offs are different as well.