anchor.diff:  class AllBooksScreen extends StatelessWidget {
     @override
     Widget build(BuildContext context) {
  -    return Center(
  -      child: Text('All Books'),
  +    return Scaffold(
  +      body: Center(
  +        child: Text('All Books'),
  +      ),
       );
     }
   }
  
positive.code:   }
  }
  
  class BooksScreen extends StatefulWidget {
    final AppPage appPage;
    final ValueChanged<int> onTabSelected;
  
    BooksScreen({
      required this.appPage,
      required this.onTabSelected,
    });
  
    @override
    _BooksScreenState createState() => _BooksScreenState();
  
negative.code:   @override
    final GlobalKey<NavigatorState> navigatorKey;
    final AppState _appState = AppState();
    late final OverlayEntry _overlayEntry =
    OverlayEntry(builder: _overlayEntryBuilder);
  
    BookRouterDelegate() : navigatorKey = GlobalKey<NavigatorState>() {
      _appState.addListener(handleAppStateChanged);
    }
  
    void handleAppStateChanged() {
      notifyListeners();
      _overlayEntry.markNeedsBuild();
    }
  
    @override
  