bug#7540: 24.0.50; cursor annoyances during scrolling on mac os x

[Hagmonk]

On modern versions of OS X there are no up and down buttons for scroll bars 
(for the last 7 or 8 years? since … 10.7.0?). Verification of this exact issue 
is therefore pretty involved, it would involve running up Emacs 25 on a machine 
compatible with that old OS version. 

Using Emacs 25.0.92.2 on OS X 10.11.3, clicking in the scroll bar area does not 
jump in any unexpected way.

Suggest returning to the reporter for reproduction on Emacs 25 and whatever 
version of OS X they use now, given the low severity of this issue.