bug#18923: Alternative scrolling model

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Date: Sun, 02 Nov 2014 21:35:44 -0500
> Cc: 18923@debbugs.gnu.org
> 
> >> BTW, have you looked at the C code of scroll-up/down at all?
> >> I'm not familiar with it, but it does do pixelwise scrolling to some
> >> extent as well (tho IIUC only for really tall lines such as those with
> >> images), so I'm curious to know exactly how the two compare.
> > I have tried different settings, but they haven't quite worked for me.
> > But I definitely need to become familiar with the existing
> > implementation.
> 
> I don't think the behavior you're looking for can be obtained from the
> current C code, indeed, but maybe the needed changes are limited.

The current implementation only uses vscroll for partially visible
screen lines (a.k.a. "glyph rows").  This is by design (and consistent
with what line-move does), but we can decide to change that, if that's
what people want.