Bottom navigation with nested navigators

import 'package:flutter/material.dart';

class TabSpec {
  const TabSpec(this.label, this.icon, this.root);
  final String label;
  final IconData icon;
  final WidgetBuilder root; // first screen of the tab
}

class TabbedShell extends StatefulWidget {
  const TabbedShell({super.key, required this.tabs});
  final List<TabSpec> tabs;

  @override
  State<TabbedShell> createState() => _TabbedShellState();
}

class _TabbedShellState extends State<TabbedShell> {
  late final List<GlobalKey<NavigatorState>> _keys =
      List.generate(widget.tabs.length, (_) => GlobalKey<NavigatorState>());
  int _index = 0;

  void _select(int i) {
    if (i == _index) {
      // Re-tapping the active tab pops it back to its root screen.
      _keys[i].currentState?.popUntil((r) => r.isFirst);
    } else {
      setState(() => _index = i);
    }
  }

  @override
  Widget build(BuildContext context) {
    final canPopTab = _keys[_index].currentState?.canPop() ?? false;
    return PopScope(
      // Let the system pop the app only when the active tab is at its root.
      canPop: !canPopTab && _index == 0,
      onPopInvokedWithResult: (didPop, _) {
        if (didPop) return;
        final nav = _keys[_index].currentState;
        if (nav != null && nav.canPop()) {
          nav.pop();
        } else {
          setState(() => _index = 0);
        }
      },
      child: Scaffold(
        body: IndexedStack(
          index: _index,
          children: [
            for (var i = 0; i < widget.tabs.length; i++)
              Navigator(
                key: _keys[i],
                onGenerateRoute: (_) =>
                    MaterialPageRoute(builder: widget.tabs[i].root),
              ),
          ],
        ),
        bottomNavigationBar: NavigationBar(
          selectedIndex: _index,
          onDestinationSelected: _select,
          destinations: [
            for (final t in widget.tabs)
              NavigationDestination(icon: Icon(t.icon), label: t.label),
          ],
        ),
      ),
    );
  }
}