bug#56393: Actually fix the long lines display bug

[Gregory Heytings]

> > The default auto-repeat rate on my system is 25.  I tried with 30, 35 
> > and 40, Emacs is still as responsive.  With 45 your recipe works, I see 
> > some chokes.  Why is that a problem?
>
> Because I think we should understand the reason.  I suspect the buffer 
> doesn't get narrowed or something.

Okay, I'll see if I can find something interesting.