bug#5654: Even slightest scrollbar move warping point

[David De La Harpe Golden]

Don't actually use the scrollbar very much, so not presently sure if 
this was just always the way emacs acted and I simply didn't notice, but 
right now (23.1.93), if you "move the scrollbar" a tiny bit (not enough 
to actually visibly scroll the buffer), the point warps to the top of 
the window.

recipe:

* emacs -Q on X11, with gtk+ (debian libgtk 2.18.6-1) toolkit scrollbars:

place point somewhere other than start of buffer. (if you've just done 
emacs -Q, it'll probably be at the end of the *scratch* buffer anyway).

mouse-1 down on scrollbar. move even a tiny amount, not enough to 
scroll. If you've just done emacs -Q, it'll already be at the top, so 
you could just push it against the top, which you might not expect to do 
anything, really.  Point warps to window start.  This happens before
you mouse-1 up.


* emacs -Q on X11, with lucid with toolkit scrollbars: (i.e. 90s-looking 
3d-effect scrollbar):

same as gtk+

* emacs -Q on X11, with lucid without toolkit scrollbars (i.e. flat 
looking scrollbar):

no warp on mouse-1, but of course mouse-1 means something different for 
these scrollbars - there is an immediate warp, but I think that's also 
different since these scrollbars immediately scroll to a location 
corresponding to the position on the scrollbar you mouse-2 down on. So 
this case might be irrelevant and it's only something that's happening 
for toolkit scrollbars.