I tried in different ways, using the global key to check for the presence of context, using canpol from context, also using local variables as described in the example below. In all cases, there are nuances, for example, if, following the example below, you run and poke between tabs, then at what point does the exception multiple global key happen, sometimes the sheet simply does not have time to close, so I tested and set future delayed so that it could close. But the problem with multiple global key remains relevant, and I don’t like this method, maybe there is some official solution? I didn’t find it in the examples.

Steps to reproduce

You need to copy the example from go_router using StatefulShellRoute.
Add an additional tab in the lower bar as a Menu, by clicking on which the Modal Bottom Sheet should open.
When switching between tabs, it is not possible to close the bottom sheet automatically.

Expected result:

When switching between tabs, it should work as it does, but if you click on Menu, then the modal bottom sheet should open, where the previous screen will be behind, from where he clicked on Menu.
After that, when you click on another tab, the bottom sheet should close.

Code:

import 'package:flutter/material.dart';
import 'package:go_router/go_router.dart';

final GlobalKey<NavigatorState> _rootNavigatorKey = GlobalKey<NavigatorState>(debugLabel: 'root');
final GlobalKey<NavigatorState> _sectionANavigatorKey = GlobalKey<NavigatorState>(debugLabel: 'sectionANav');

void main() {
  runApp(NestedTabNavigationExampleApp());
}

class NestedTabNavigationExampleApp extends StatelessWidget {
  NestedTabNavigationExampleApp({super.key});

  final GoRouter _router = GoRouter(
    navigatorKey: _rootNavigatorKey,
    initialLocation: '/a',
    routes: <RouteBase>[
      StatefulShellRoute.indexedStack(
        builder: (BuildContext context, GoRouterState state, StatefulNavigationShell navigationShell) {
          return ScaffoldWithNavBar(
            navigationShell: navigationShell,
          );
        },
        branches: <StatefulShellBranch>[
          StatefulShellBranch(
            navigatorKey: _sectionANavigatorKey,
            routes: <RouteBase>[
              GoRoute(
                path: '/a',
                builder: (BuildContext context, GoRouterState state) => const TabScreen(
                  text: 'A',
                ),
              ),
            ],
          ),
          StatefulShellBranch(
            routes: <RouteBase>[
              GoRoute(
                path: '/b',
                builder: (BuildContext context, GoRouterState state) => const TabScreen(
                  text: 'B',
                ),
              ),
            ],
          ),
          StatefulShellBranch(
            routes: <RouteBase>[
              GoRoute(
                path: '/menu',
                pageBuilder: (context, state) => BottomSheetPage(
                  key: state.pageKey,
                  child: const _MenuBottomSheetBody(),
                ),
              ),
            ],
          ),
          StatefulShellBranch(
            routes: <RouteBase>[
              GoRoute(
                path: '/c',
                builder: (BuildContext context, GoRouterState state) => const TabScreen(
                  text: 'C',
                ),
              ),
            ],
          ),
          StatefulShellBranch(
            routes: <RouteBase>[
              GoRoute(
                path: '/d',
                builder: (BuildContext context, GoRouterState state) => const TabScreen(
                  text: 'D',
                ),
              ),
            ],
          ),
        ],
      ),
    ],
  );

  @override
  Widget build(BuildContext context) => MaterialApp.router(
        routerConfig: _router,
      );
}

class TabScreen extends StatelessWidget {
  const TabScreen({
    required this.text,
    super.key,
  });

  final String text;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Text(
          text,
          style: const TextStyle(fontSize: 24),
        ),
      ),
    );
  }
}

class _MenuBottomSheetBody extends StatelessWidget {
  const _MenuBottomSheetBody();

  @override
  Widget build(BuildContext context) {
    return Material(
      color: Colors.transparent,
      child: DecoratedBox(
        decoration: const BoxDecoration(
          color: Colors.white,
          borderRadius: BorderRadius.only(
            topLeft: Radius.circular(16),
            topRight: Radius.circular(16),
          ),
        ),
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: <Widget>[
            ListTile(
              title: const Text('Option 1'),
              onTap: () {},
            ),
            ListTile(
              title: const Text('Option 2'),
              onTap: () {},
            ),
          ],
        ),
      ),
    );
  }
}

class ScaffoldWithNavBar extends StatefulWidget {
  const ScaffoldWithNavBar({
    required this.navigationShell,
    Key? key,
  }) : super(key: key ?? const ValueKey<String>('ScaffoldWithNavBar'));

  final StatefulNavigationShell navigationShell;

  @override
  State<ScaffoldWithNavBar> createState() => _ScaffoldWithNavBarState();
}

class _ScaffoldWithNavBarState extends State<ScaffoldWithNavBar> with TickerProviderStateMixin {
  late final TabController _tabController = TabController(length: 5, vsync: this);

  @override
  void dispose() {
    super.dispose();
    _tabController.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: widget.navigationShell,
      bottomNavigationBar: TabBar(
        controller: _tabController,
        indicator: TopIndicator(),
        tabs: const <Tab>[
          Tab(icon: Icon(Icons.work), text: 'A'),
          Tab(icon: Icon(Icons.work), text: 'B'),
          Tab(icon: Icon(Icons.menu), text: 'Menu'),
          Tab(icon: Icon(Icons.tab), text: 'C'),
          Tab(icon: Icon(Icons.settings), text: 'D'),
        ],
        onTap: (int index) async {
          if (index == 2) {
            setState(() {
              GoRouter.of(context).push('/menu').then((_) {
                _tabController.animateTo(widget.navigationShell.currentIndex);
              });
            });
          } else {
            widget.navigationShell.goBranch(
              index,
              initialLocation: index == widget.navigationShell.currentIndex,
            );
          }
        },
      ),
    );
  }
}

class TopIndicator extends Decoration {
  @override
  BoxPainter createBoxPainter([VoidCallback? onChanged]) => _TopIndicatorBox();
}

class _TopIndicatorBox extends BoxPainter {
  @override
  void paint(Canvas canvas, Offset offset, ImageConfiguration cfg) {
    final paint = Paint()
      ..shader = const RadialGradient(
        colors: [
          Colors.black,
          Colors.black,
        ],
      ).createShader(
        Rect.fromCircle(
          center: offset,
          radius: 0,
        ),
      )
      ..strokeWidth = 2
      ..isAntiAlias = true
      ..strokeCap = StrokeCap.square;

    canvas.drawLine(
      Offset(offset.dx, 0.5),
      Offset(cfg.size!.width + offset.dx, 0.5),
      paint,
    );
  }
}

class BottomSheetPage extends Page {
  const BottomSheetPage({
    required this.child,
    this.showDragHandle = true,
    this.useSafeArea = false,
    super.key,
    super.restorationId = 'bottomSheet',
  });
  final Widget child;
  final bool showDragHandle;
  final bool useSafeArea;

  @override
  Route createRoute(BuildContext context) => ModalBottomSheetRoute(
        settings: this,
        isScrollControlled: true,
        showDragHandle: showDragHandle,
        useSafeArea: useSafeArea,
        backgroundColor: Colors.transparent,
        builder: (context) => (ModalRoute.of(context)!.settings as BottomSheetPage).child,
      );
}

Video:

https://github.com/user-attachments/assets/88869c00-f531-48e7-a4f1-f0b42cb90841