If a given coordinate is a function, it is called at the moment the scrolling action begins (and therefore the up-to-date value of the coordinate is needed), and is passed a single argument: the scrolling element that is being scrolled.
If a given coordinate is a function, it is called at the moment the scrolling action begins (and therefore the up-to-date value of the coordinate is needed), and is passed a single argument: the scrolling element that is being scrolled.