The total scroll extent of the gap being absorbed by the SliverOverlapAbsorber.
This is updated during the layout of the SliverOverlapAbsorber. It should not change at any other time. No notifications are sent when it changes; clients (e.g. SliverOverlapInjectors) are responsible for marking themselves dirty whenever this object sends notifications, which happens any time the SliverOverlapAbsorber might subsequently change the value during that layout.
double get scrollExtent => _scrollExtent;