Emacs 21: select text, scroll, click causes random jump in buffer

[J Rulnick]

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In GNU Emacs 21.1.1 (i686-pc-linux-gnu, X toolkit)
 of 2002-11-22 on osprey
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: nil
  locale-coding-system: nil
  default-enable-multibyte-characters: t

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

The following problem is present in Emacs 21.x and is 100% repeatable
in my tests.  It occurs with all "--with-x-toolkit=lucid|motif" and
"--with[out]-toolkit-scroll-bars" combinations.  It occurs with the
Debian package of Emacs 21, and with Emacs built from the gnu.org
source.  It occurs with or without use of the xaw3d and/or lesstif
libraries.  It occurs in Emacs 21.1 and 21.2, but NOT in Emacs 20.x.
Hence the problem appears to have been introduced in Emacs 21.

To produce the problem, follow these steps.

1. emacs -q FILE

   (make sure FILE is a text file large enough to require scrolling a
   few frames)

2. use mouse to select text

   (double-click, triple-click, or click-and-drag a small region of
   text)

3. click a mouse button in the scroll bar area once or twice to
   scroll the buffer contents, far enough so that the marked text
   moves (up or down) outside the viewable region

4. now click mouse button 1 once on any of the text in the viewable
   region, and notice that the point (cursor) jumps to a different
   point in the buffer, NOT the text just clicked

In summary: using the mouse, select some text, scroll to a different
part of the file, then click inside the buffer.  The "jump" appears to
be random, and repeated trials indicate it may be a "double-jump,"
i.e., the window-start may appear to change twice rapidly after the
click.

Please don't hesitate to contact me for more details.  All comments,
suggestions, and questions welcome.

Please cc: emacs-bug@rulnick.com in any follow-up.

Thank you.

John

P.S.  The problem was tested most recently using Linux kernel 2.4.17
and XFree86 4.2.1, but was also experienced under earlier versions of
Linux and XFree86, all using Debian GNU/Linux on lightly-loaded i386
architecture machines with 256 MB or more RAM.

Recent input:
C-x C-f I N S T A L L <return> <down-mouse-1> <mouse-1> 
<double-down-mouse-1> <double-mouse-1> <triple-down-mouse-1> 
<triple-mouse-1> <down-mouse-1> <mouse-1> <down-mouse-1> 
<mouse-1> <escape> x r e p o r t - e m a c s - b u 
g <return>

Recent messages:
(src/emacs -q)
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