bug#14819: Scrolling page up-page down with scroll bar does not work

[Angelo Graziosi]

To reproduce:

$ emacs -Q &
C-x C-f A_LONG_TEXT_FILE

The scroll bar is on the right and it has an UP arrow and a DOWN arrow. 
Clicking with mouse-1 on these arrows, scrolls the buffer one line at 
time (up or down), and this is OK.

Now, clicking just above the DOWN arrow or just below the UP arrow 
should scroll the buffer a *page* at time (down or up), instead it only 
moves the scroll bar cursor.. :-(


Ciao,
Angelo.