The GtkViewport object is only used alongside a
GtkScrolledWindow, and provides scrolling
capability where the child widget being added to a scrolled window is
not otherwise scrollable.
It is not possible in most cases to use it outside this context, as its
adjustment values are set internally by the child's sizing policies and
can neither be overridden manually nor used directly. Note that
directional keybindings in a non-scrolling aware widget are not usually
bound to any adjustment value by default; the
GtkViewport simply allows the use of scrollbars
with such a widget. The exception that proves this rule is
GtkList, which is set up so that it can be made
to be scrollable via the keyboard if it is added to a
GtkViewport.
In most circumstances, it is easier to construct the viewport object by
using the convenience method
add_with_viewport()
rather than by using its own constructor.
