Scrolling down fails to refresh.

[Bernard A Badger]

The following is a screen capture (Alt-Print Scrn) of Emacs as it scrolls badly
and does not
refresh properly.  
GNU Emacs 21.2.1 (i386-msvc-nt5.0.2195) of 2002-03-19 on buffy
If you cannot see the embedded picture, here's a description of the problem.

When I scroll down the window (Pull scrollbar down, that is) and there is a
"Stay on Top"
window overlapping the bottom of the emacs window
(such as Rational Purify's "Purify Instrumentation" progress window, 
or the Windows "Task Manager" window),  
the graphics of the obscuring window are copied up as we scroll.  
Sometimes they are erased in the end, sometimes not.
(I say "scroll down" when the window goes down the file, the corresponding
text appears to go up the screen.)

This can occur multiple times, until there is a "chatter" of wrongly-scrolled
window parts.

I suspect this has something to do with the incorrect scrolling that occurs
anyway.  
This is when the size of the scroll bar is re-computed based on the number of
characters 
on-screen.  It should really be based on the number of lines.  I think that in
the end it is, and
it's the temporary "approximate" scrolling that is to blame.

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