maxScrollObstructionExtent property Null safety
The maximum extent by which this sliver can reduce the area in which content can scroll if the sliver were pinned at the edge.
Slivers that never get pinned at the edge, should return zero.
A pinned app bar is an example for a sliver that would use this setting: When the app bar is pinned to the top, the area in which content can actually scroll is reduced by the height of the app bar.
final double maxScrollObstructionExtent;