Part of the 2D scrolling proposal: flutter.dev/go/2D-Foundation

The AppBar scrolled under behavior assumes any horizontal scroll notification means a vertical axis can't possibly be be currently scrolled under the AppBar. In 2D, this will not longer be true.
For 2D friendliness, it should just not do anything if a horizontal scroll notification passes the notification predicate, especially since it is already configured to only apply to only the vertical axis.