hasScrollBody property Null safety
Indicates whether the child has a scrollable body, this value cannot be null.
Defaults to true such that the child will extend beyond the viewport and scroll, as seen in NestedScrollView.
Setting this value to false will allow the child to fill the remainder of the viewport and not extend further. However, if the SliverConstraints.precedingScrollExtent and/or the child's extent exceeds the size of the viewport, the sliver will defer to the child's size rather than overriding it.
final bool hasScrollBody;