bug#18143: Vertical scroll bar does not work with Image files

[Angelo Graziosi]

When an image file (jpg, png..) is displayed greater than the frame 
dimensions, a way to explore it is using the arrow keys (up, down, left 
and right).

With the introduction of horizontal scroll bar, it is very nice to use 
it in place of L-R arrow keys: the HSB works fine here!

This does not occur with vertical scroll bar. It has the full height of 
the frame and cannot be moved U-D. So the only way to explore the image 
vertically is to use the U-D keys.

I see this both with MSYS2/MinGW64 and Cygwin builds of Emacs trunk.


Ciao,
 Angelo.