read-only
Called to determine the offset to which scrolling should snap, when handling a fling.
This callback, if set, will be called with the offset that the Scrollable would have scrolled to in the absence of this callback, and a Size describing the size of the Scrollable itself.
The callback's return value is used as the new scroll offset to aim for.
If the callback simply returns its first argument (the offset), then it is as if the callback was null.