An IndexedWidgetBuilder that's called when tabs become active.
The widgets built by IndexedWidgetBuilder are typically a
CupertinoTabView in order to achieve the parallel hierarchical
information architecture seen on iOS apps with tab bars.
When the tab becomes inactive, its content is cached in the widget tree
Offstage and its animations disabled.
Content can slide under the tabBar when they're translucent.
In that case, the child's BuildContext's MediaQuery will have a
bottom padding indicating the area of obstructing overlap from the
final IndexedWidgetBuilder tabBuilder;