Enbales or disables last-line scrolling, or gets the current enable state. If last-line scrolling is enabled, then an editor displayed 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, an editor can only be scrolled until the last line is at the bottom (or top) of the canvas.
Returns #t if last-line scrolling is enabled, #f otherwise.
If on? is #f, last-line scrolling is disabled, otherwise it is enabled.