Interface between an adjustment and a scrollbar widget.
Return offset incremented by one step
If range > number of scroll bar steps then step>=1.
adj#mouse_scroll offset computes the scroll bar based on a click
offset units from the top/left
Configure how the size of the scrollbar is calculated.
`fixed x sets the size to x.
`dynamic 0 sets the size to reflect the ratio between the range and scroll window size.
`dynamic x (x>0) interprets
x as the viewable size and sets the size of the scroll bar to reflect the amount of content displayed relative to range.
Configure how a mouse coordinate is converted to a scroll bar offest.
`middle sets the middle of the scrollbar to the position clicked.
`ratio computes the offset relative to the scroll bar and scroll window sizes, with a 10% deadzone at the extremities.
`middle mode if the scroll bar size is less than half the window size and
Set the maximum scroll bar size (default: scroll window size
method on_scrollbar_change : ?switch:LTerm_widget_callbacks.switch -> (unit -> unit) -> unit
on_scrollbar_change ?switch f calls f when the scrollbar is changed and needs to be re-drawn.