bug#56393: Actually fix the long lines display bug

[Gregory Heytings]

> > The first takes several seconds, the second is almost instantaneous.
>
> Ah, I misunderstood what you were saying.
>
> But you originally talked about C-n and vertical-motion, not about C-s.

Yes, what I meant is that the effect is visible both with C-n / C-p and 
with C-s.  And probably with other commands.