[21.1]: non-nil scroll-preserve-screen-position doesn't with thum bing

[Marshall, Simon]

In GNU Emacs 21.1.1 (sparc-sun-solaris2.6, GNU/LessTif Version 2.1
Release 0.93.3)
 of 2001-12-03 on ashanti
configured using `configure  --prefix=/rvcarma/marshals/slash/usr/local
--x-includes=/usr/openwin/include:/rvcarma/marshals/slash/usr/local/incl
ude:/rvcarma/marshals/slash/usr/local/include/X11
--x-libraries=/usr/openwin/lib:/rvcarma/marshals/slash/usr/local/lib:/rv
carma/marshals/slash/usr/local/lib/X11 --with-x-toolkit=motif'
Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: en_UK
  value of $LC_CTYPE: en_UK
  value of $LC_MESSAGES: C
  value of $LC_MONETARY: en_UK
  value of $LC_NUMERIC: en_UK
  value of $LC_TIME: en_UK
  value of $LANG: en_UK
  locale-coding-system: iso-latin-1
  default-enable-multibyte-characters: t

Setting scroll-preserve-screen-position to non-nil has no effect when
scrolling with the scrollbar thumb.  It does not move point except to
keep point within the window.  (It does have an effect when scrolling
with the scrollbar arrows.)

Shouldn't it always move point to keep it in the middle of the window?