bug#5177: 23.1.90; scrollbar thumb stops working after being dragged

Please write in English if possible, because the Emacs maintainers
usually do not have translators to read other languages for them.

Your bug report will be posted to the [email protected] mailing 
list.

Please describe exactly what actions triggered the bug
and the precise symptoms of the bug.  If you can, give
a recipe starting from `emacs -Q':

1. Start the Gtk+ build of emacs.
2. Visit a file too big to fit in the window.
3. Drag the (vertical) scrollbar thumb.
4. Scroll through the buffer using the mouse wheel or keystrokes.

The scrollbar thumb stays fixed and no longer shows the relative
position in the buffer.  Clicking in an empty area of the scrollbar
restores the correct behavior.

If Emacs crashed, and you have the Emacs process in the gdb debugger,
please include the output from the following gdb commands:
     `bt full' and `xbacktrace'.
For information about debugging Emacs, please read the file
/usr/share/emacs/23.1.90/etc/DEBUG.


In GNU Emacs 23.1.90.1 (i686-pc-cygwin, GTK+ Version 2.18.3)
  of 2009-12-10 on markov
Windowing system distributor `The Cygwin/X Project', version 11.0.10701000
configured using `configure 
'--srcdir=/usr/src/emacs-23.1.90-1/src/emacs-23.1.90' '--prefix=/usr' 
'--exec-prefix=/usr' '--bindir=/usr/bin' '--sbindir=/usr/sbin' 
'--libexecdir=/usr/lib' '--datadir=/usr/share' '--localstatedir=/var' 
'--sysconfdir=/etc' '--datarootdir=/usr/share' 
'--docdir=/usr/share/doc/emacs' 'CC=gcc' 'CFLAGS=-O2 -pipe ' 'LDFLAGS=' 
'LIBS=' 'CPPFLAGS=''

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.UTF-8
   value of $XMODIFIERS: nil
   locale-coding-system: utf-8-unix
   default enable-multibyte-characters: t

Major mode: Lisp Interaction

Minor modes in effect:
   tooltip-mode: t
   mouse-wheel-mode: t
   tool-bar-mode: t
   menu-bar-mode: t
   file-name-shadow-mode: t
   global-font-lock-mode: t
   font-lock-mode: t
   blink-cursor-mode: t
   global-auto-composition-mode: t
   auto-composition-mode: t
   auto-encryption-mode: t
   auto-compression-mode: t
   line-number-mode: t
   transient-mark-mode: t

Recent input:
<help-echo> <help-echo> M-x r e p o r t - e m <tab>
<return>

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Load-path shadows:
None found.

Features:
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password-cache mm-decode mm-bodies mm-encode mailcap mail-parse rfc2231
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time-date mm-util mail-prsvr gmm-utils wid-edit mailheader canlock sha1
hex-util hashcash mail-utils warnings emacsbug sendmail regexp-opt
tooltip ediff-hook vc-hooks lisp-float-type mwheel x-win x-dnd
font-setting tool-bar dnd fontset image fringe lisp-mode register page
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emacs)