bug#12811: 24.3.50; `scroll-up/down-aggressively' don't seem to work as expected

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sun, 11 Nov 2012 17:55:59 +0100
> From: Dani Moncayo <dmoncayo@gmail.com>
> Cc: 12811@debbugs.gnu.org
> 
> > Emacs doesn't count in lines, it counts in pixels.  And since a line
> > has a finite size, where exactly those 70% end is not well determined
> > (is it the top of the line, the bottom of the line, somewhere in
> > between?).
> 
> I don't see the point of using pixel as unit of measure here.  The
> unit should be the line, because the problem to solve here can
> (should) be expressed in terms of lines; going down to the pixel level
> is unnecessary and confusing, IMO.

You forget about variable fonts.  One line can be very large, another
very small.  Scrolling must still do something reasonable with them.

> So, given a window which is W lines high

Since lines can have different heights, "W lines high" is not well
defined.  In extreme case, a window can be 1 line high, but scrolling
it bring 20 lines into view.

It is impossible to have reasonable scrolling if you go by lines.