Bad redisplay

[David Kastrup]

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Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

Actually, for people playing around with the Emacs display engine, I
recommend installing preview-latex
<URL:http://preview-latex.sourceforge.net> and checking out the
behavior of the example document circ.tex when using a large font (I
use 24pixels fixed, I believe).  This produces a buffer with a
serious number of images, several of them larger than the typical
Emacs window, several at least pretty large.

Here is the latest problem: when the cursor is below the image and I
use cursor-up to move behind it, a copy of the image is written to
below the image as if Emacs was planning to scroll a significant
amount, then it doesn't scroll that much after all.  Two copies of the
image are then on the screen (don't mind the incomplete cursor
display: that's at a place occupied by the screenshot application.
Don't ask me why only the cursor display is affected and not the
whole image).

Screenshot-2.png
Description: before cursor up

Screenshot-3.png
Description: after cursor up

In GNU Emacs 21.3.50.21 (i686-pc-linux-gnu, GTK+ Version 2.2.1)
 of 2004-04-20 on lola.goethe.zz
configured using `configure '--prefix=/usr/local/emacs-21' '--with-gtk''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: en_US
  locale-coding-system: iso-latin-1
  default-enable-multibyte-characters: t

Major mode: LaTeX

Minor modes in effect:
  reftex-mode: t
  auto-compression-mode: t
  desktop-save-mode: t
  display-time-mode: t
  iswitchb-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  unify-8859-on-encoding-mode: t
  line-number-mode: t

Recent input:
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <up> <down> <down> <up> <down> <down> <up> <down> 
<down> <up> <up> <up> <down> <up> <up> <up> <down> 
<down> <down> <up> <down> <down> <up> <down> <down> 
<help-echo> <help-echo> <up> M-x g n u s C-g M-x g 
n u s <return> n n <return> 3 <return> n E q SPC q 
q y M-m C-g M-x r e p o r t - e m a <tab> <return>

Recent messages:
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-- 
David Kastrup, Kriemhildstr. 15, 44793 Bochum