bug#32002: 24.4; Scroll bar start, end not correct

[Eli Zaretskii <eliz@gnu.org>]

> From: Mike Kupfer <mkupfer@alum.berkeley.edu>
> cc: Andrew Kurn <kurn@sfu.ca>, 32002@debbugs.gnu.org
> Date: Fri, 13 Jul 2018 21:56:40 -0700
> 
> With
> 
>   (setq scroll-bar-adjust-thumb-portion nil)
> 
> the thumb takes up the entire track.  (I'm doing this testing with a
> theme that lacks steppers.)  Moving the mouse down (while holding
> button1) does not move the thumb, but it does cause the text to scroll
> up.  Moving the mouse up (while holding button1) does not cause the text
> to reappear, even if I move the mouse all the way to the top of the
> thumb.
> 
> > Does Emacs receive
> > scroll-bar scroll events when you do that (you can verify that with
> > "C-h l")?
> 
> Yes (at least for my test scenario).

Which part of scroll-bar-toolkit-scroll gets executed when you move
the thumb up, after scrolling the entire text out of the window?  Is
it this:

	 ((eq part 'up)
	  (scroll-up -1))

or this:

	 ((eq part 'handle)
	  (scroll-bar-drag-1 event))))

My guess is the latter, in which case I think the problem is in
scroll-bar-drag-1.  Can you spot it?  I think it calculates the new
starting point of the window incorrectly, because it doesn't take into
account the special situation with scroll-bar-adjust-thumb-portion in
this particular case.

Thanks.