If last-line scrolling is enabled, then a buffer viewed in this canvas can be scrolled so that the last line of text is at the top of the canvas (or bottom of the canvas when bottom-based scrolling is enabled; see scroll-with-bottom-base). By default, a buffer can only be scrolled until the last line is at the bottom (or top) of the canvas.
If on? is #f, last-line scrolling is disabled, otherwise it is enabled.