bug#18923: Alternative scrolling model

[Eli Zaretskii]

> From: E Sabof <esabof@gmail.com>
> Cc: 18923@debbugs.gnu.org
> Date: Sun, 02 Nov 2014 18:25:28 +0000
> 
> >  . the auto-window-vscroll variable
> 
> I think the variable would no longer be useful with this approach.

In that case, we should deprecate it.  I see it being used at least in
Gnus.

> > There's also a bug when scrolling near the end of buffer: the result
> > is that the cursor us shown on a line beyond EOB, which should never
> > happen.
> 
> I've noticed this. The point flashes at a wrong position, and then goes
> back to "legal" position.

It doesn't go back on my system, unless I move point in some way.

> I thought this was a display engine bug.

It could be that.

> I'm aware of some inefficiencies in my code. My guess probably doesn't
> mean much, but maybe something along 2X could be achieved.

Speeding up twice might be all we need to get a reasonably fast
implementation.