Base class for GtkHScrollbar and GtkVScrollbar
The <gtk-scrollbar> widget is an abstract base class for
<gtk-hscrollbar> and <gtk-vscrollbar>. It is not very useful in
itself.
The position of the thumb in a scrollbar is controlled by the scroll
adjustments. See <gtk-adjustment> for the fields in an adjustment - for
<gtk-scrollbar>, the "value" field represents the position of the
scrollbar, which must be between the "lower" field and "upper - page_size." The
"page_size" field represents the size of the visible scrollable area. The
"step_increment" and "page_increment" fields are used when the user asks to step
down (using the small stepper arrows) or page down (using for example the
PageDown key).