bug#12419: Mouse click changes layout

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 26 Sep 2012 15:44:43 +0200
> From: martin rudalics <rudalics@gmx.at>
> CC: occitan@esperanto.org, 12419@debbugs.gnu.org
> 
>  > I think the loop isn't needed for the application you have in mind.
>  > But if it is, can you show a concrete use case where just using that
>  > function gives bad results for measuring pixel distance between buffer
>  > positions?
> 
> I didn't write that loop (IIRC Kenichi Handa wrote it).  The sheer
> presence of it however seems to indicate that `pos-visible-in-window-p'
> might have given an off-by-one-line answer when `point' is on the last
> line of a window, at least back in 2000.  Maybe this a non-issue now.

I think that loop wants to make sure the last line is fully visible;
pos-visible-in-window-p does not return nil for partially-visible
lines.  I don't think partial visibility is an issue in the context of
finding pixel distance between two buffer positions.