If non-null, forces the children to have the same extent as the given widget in the scroll direction.
Specifying an prototypeItem is more efficient than letting the children determine their own extent because the scrolling machinery can make use of the foreknowledge of the children's extent to save work, for example when the scroll position changes drastically.
- SliverPrototypeExtentList, the sliver used internally when this property is provided. It constrains its box children to have the same extent as a prototype item along the main axis.
- The itemExtent property, which allows forcing the children's extent to a given value.
- The itemExtentBuilder property, which allows forcing the children's extent to be the value returned by the callback.
final Widget? prototypeItem;