the adjustment.lower attribute is the
minimum value of the scroll region
the adjustment.upper attribute is the
maximum value of the scroll region
the adjustment.value attribute
represents the position of the scrollbar, which must be between
adjustment.lower and adjustment.upper
- adjustment.page_size
the adjustment.page_size attribute
represents the size of the visible scrollable area
the adjustment.step_increment attribute
is the distance to scroll when the small stepper arrows are clicked
the adjustment.page_increment attribute
is the distance to scroll when the Page Up or Page
Down keys are pressed