bug#56393: Actually fix the long lines display bug

[Gregory Heytings]

> > Which leaves me with something to think about, of course: when 
> > redisplay sets the flag in the buffer, then there's of course a time 
> > before that. And the question is what can happen in that interval. 
> > But not today. I'll come back to you.
>
> So far, I can't think of something specifically catastrophic happening 
> in the interval between a buffer being loaded, let's say, and it being 
> marked with the flag.
>
> I think it's the same situation we talked about previously wrt a buffer 
> that is never or not yet displayed:
>
> Pretty much anything could potentially happen as soon as an iterator is 
> used.  The reason for this is that iterators must know faces and other 
> stuff, let's simply say they must know text properties and overlays. And 
> that can call Lisp, and then all bets are off.
>
> If that happens in practice, how often it happens, or what else, I don't 
> know.
>
> What's the worst thing that can happen?  Slowness, I guess.  I can't 
> think of something going seriously down the drain ATM.

Okay, now I see what you mean.  As far as I can see (but I could be 
wrong), there are no significant slowdowns between the moment the file is 
opened and the moment it is displayed.  If such slowdowns exist, they are 
a separate problem, and will be dealt with when they will be reported.

> I guess that's about what I can continute to the whole matter.
>
> Nice work.  THanks!

Thanks! 😃