An IndexedWidgetBuilder that's called when tabs become active.
The widgets built by IndexedWidgetBuilder is typically a CupertinoTabView
in order to achieve the parallel hierarchies information architecture seen
on iOS apps with tab bars.
When the tab becomes inactive, its content is still 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
Must not be null.
final IndexedWidgetBuilder tabBuilder