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

[martin rudalics <rudalics@gmx.at>]

 > Here's a possible way to test: if you have a mouse with multiple
 > buttons, right-click in the scrollbar above or below the thumb (slider).
 > Based on my testing, that appears to be equivalent to clicking on a
 > stepper button (with GTK3).

The areas above and below the thumb (or slider), if they exist, serve
for jumping to the buffer position corresponding to the position
clicked at.  It's only natural that this results in a "jump" - where
calculating the new position of an object in a window relates the
position of the mouse cursor on the associated scroll bar and the size
and position of the thumb to the overall size of the object.

The arrow keys serve a completely different purpose: They support line
by line (or column by column) scrolling and are, in practice, used
only when the area reserved for the scroll bar is too small to
accommodate a thumb or the user has no mouse wheel in order to scroll
a window.  Pressing an arrow key results in a "step" - where the
calculation of the new position of an object in a window is based on
its old position plus/minus a fixed number of lines/columns or pixels.

Please keep these two concepts apart when suggesting how people should
test the behavior of their scroll bars.

Thanks, martin