Internal: b/144318472

Consider a CustomScrollView with a SliverAppBar and a SliverList and the user wants a scrollbar showing the position inside the SliverList's contents only.

Let the SliverAppBar report SliverGeometry describing its obstructing and max obstructing extends. Let that get aggregated by the scrollable and bubbled up with a scroll notification so the scrollbar can start below the obstructing area.