bug#10304: 24.0.92: display bug

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Magne Ingebrigtsen <larsi@gnus.org>
> Cc: 10304@debbugs.gnu.org
> Date: Tue, 02 Dec 2014 18:01:08 +0100
> 
> I've produced four screenshots instead, which should make it clearer
> what it's supposed to look like, and what's the bug.
> 
> First, I've selected the article, and it's displaying the topmost part
> of the article, and everything is fine.

Thanks, and sorry again for a _really_ long delay.

Based on the stuff you collected and the screenshots, it sounds like
Emacs doesn't clear and doesn't redraw the parts of the screen which
become corrupted, amazing as that sounds.  The only difference between
the "good" and the "bad" output is that in the latter case, for some
reason, Emacs stops clearing and redrawing screen lines below some Y
coordinate, and all the stuff which was there before that stays put
instead of being replaced by the new stuff.

I'd like to prepare an additional patch which will allow you to
collect more data that might reveal why Emacs behaves like that.  In
order to test the patch, is it possible for you to give me
instructions for how to display that article, starting from "emacs -Q"?

Thanks in advance.  I think we are making progress, albeit slowly (my
fault, not yours).