Whether the nav bar should stretch to fill the over-scroll area.
The nav bar can still expand and contract as the user scrolls, but it will
also stretch when the user over-scrolls if the stretch value is
When set to
true, the nav bar will prevent subsequent slivers from
accessing overscrolls. This may be undesirable for using overscroll-based
widgets like the CupertinoSliverRefreshControl.
final bool stretch;