disturbing, hard-to-catch redisplay bug

[John Wiegley <johnw@gnu.org>]

Well, I have seen this twice now, but I can't catch it.  If I
restart my Emacs clean, the problem doesn't appear.  It only happens
if I've been using Emacs for a while.

I did, however, at least capture it visually.  These two images show
the bug:

In the first, I am editing a list of filenames:

  http://www.gci-net.com/~johnw/root1.jpg.bz2

Now I press M-} to advance to the next whitespace line, at which
point it "appends" to the end of my lines with content from the new
buffer location:

  http://www.gci-net.com/~johnw/root2.jpg.bz2

This is not real, however, as C-l (or any other redisplaying event)
will cure it.  I *have* moved, but the text of my new location does
not start at column 0, but rather is at the end of the visual lines
from my previous location -- at least until the next redisplay.

I hope the pictures will trigger someone's memory.  As I said,
changing my environment in any significant way causes the bug to
fade away.

I only see this when my screen is split.  A one-window display does
not have the problem.  A three-window display only has the problem
in the "half frame height weight" (i.e., the display you get after
pressing C-x 2 twice from a full frame).  If I balance the
three-window display, the problem does not appear.

Very odd,
  John