Scrollbar bugs in Windows port

[Ralf Angeli <angeli@caeruleus.net>]

This is basically a follow-up to the "Scrollbars ?" thread on
emacs-pretest-bug trying to provide test cases for three of the
scrollbar-related bugs in the Windows port of Emacs mentioned in that
thread.

The following was tested with a version checked out and built
2006-05-14.  The font in effect was
-outline-Courier New-normal-r-normal-normal-16-96-120-120-c-*-iso8859-1.

The problems occur under Windows only.  A version built on GNU/Linux
with GTK scrollbars for example is unaffected.

1) If you start Emacs with `emacs -Q', open a file which is longer
   than the window, say etc/DEBUG, move point to the end of the buffer
   and click with mouse-1 on the scrollbar thumb, the buffer will be
   scrolled down by one line upon the mouse-down event.

2) If you start Emacs with `emacs -Q', open etc/DEBUG, click with
   mouse-1 on the scrollbar thumb, drag it down about two thirds of
   the scrollbar and then drag it slowly further down, the buffer will
   shift fastly and eratically up and down during dragging.

3) If you start Emacs with `emacs -Q', open etc/GNU, click with
   mouse-1 on the scrollbar thumb, drag it down to the end of the
   scrollbar and release mouse-1, the last line of the file will not
   be visible.

-- 
Ralf