bug#56393: Actually fix the long lines display bug

[Gerd Möllmann]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Gerd Möllmann <gerd.moellmann@gmail.com>
>> 
>> Is finding newlines that expensive?  (I don't remember.)
>
> It isn't, but if you have a very large buffer, finding _all_ the
> newlines in it could take time.
>
>> I mean, if someone turns that "mode" on, does he deserve to suffer a
>> little bit while that is switched on?
>
> We want to have this "mode" turned on by default.

Okay.

If that's already determined, I don't need to think about it.

>
>> I think someone with such a long line deserves to suffer.
>
> That's a bootstrap problem: when you first visit a file, you usually
> don't know whether it will have long lines.  By the time you know,
> it's many times too late.  Thus such a feature should ideally be
> active by default, unless it sometimes causes serious problems.

Ideally, right.

What I'm trying to hint at it is that there's another way to see this:

It was mentioned a number of times that only a small fraction of people
encounter that problem.  And I'd add that this small fraction encounters
the problem only in a small fraction of cases.  And I even think most of
these cases are known upfront.

So, with some kind of mode, one could turn the iterator-narrowing on
(and off!) in redisplay_window without affecting anyone who doesn't have
the problem in the first place.

Just wanted to explain that.  No need for discussions.