[go_router] Stop inactive StatefulShellRoute branches from vetoing pops

packages/go_router/lib/src/builder.dart

@@ -7,6 +7,7 @@ import 'package:flutter/widgets.dart';
 import 'configuration.dart';
 import 'logging.dart';
 import 'match.dart';
+import 'misc/active_branch_scope.dart';
 import 'misc/error_screen.dart';
 import 'misc/errors.dart';
 import 'pages/cupertino.dart';
@@ -336,30 +337,45 @@ class _CustomNavigatorState extends State<_CustomNavigator> {
             List<NavigatorObserver>? observers,
             String? restorationScopeId,
           ) {
-            return PopScope(
-              // Prevent ShellRoute from being popped, for example
-              // by an iOS back gesture, when the route has active sub-routes.
-              // TODO(LukasMirbt): Remove when minimum flutter version includes
-              // https://github.com/flutter/flutter/pull/152330.
-              canPop: match.matches.length == 1,
-              child: _CustomNavigator(
-                // The state needs to persist across rebuild.
-                key: ValueKey<GlobalKey<NavigatorState>>(navigatorKey),
-                navigatorRestorationId: restorationScopeId,
-                navigatorKey: navigatorKey,
-                matches: match.matches,
-                matchList: matchList,
-                inheritedMetadata: state.metadata,
-                configuration: widget.configuration,
-                observers: observers ?? const <NavigatorObserver>[],
-                onPopPageWithRouteMatch: widget.onPopPageWithRouteMatch,
-                // This is used to recursively build pages under this shell route.
-                errorBuilder: widget.errorBuilder,
-                errorPageBuilder: widget.errorPageBuilder,
-                requestFocus: widget.requestFocus,
-                isShellNavigator: true,
-                clipBehavior: _clipBehaviorFor(match.route, navigatorKey),
-              ),
+            final Widget navigator = _CustomNavigator(
+              // The state needs to persist across rebuild.
+              key: ValueKey<GlobalKey<NavigatorState>>(navigatorKey),
+              navigatorRestorationId: restorationScopeId,
+              navigatorKey: navigatorKey,
+              matches: match.matches,
+              matchList: matchList,
+              inheritedMetadata: state.metadata,
+              configuration: widget.configuration,
+              observers: observers ?? const <NavigatorObserver>[],
+              onPopPageWithRouteMatch: widget.onPopPageWithRouteMatch,
+              // This is used to recursively build pages under this shell route.
+              errorBuilder: widget.errorBuilder,
+              errorPageBuilder: widget.errorPageBuilder,
+              requestFocus: widget.requestFocus,
+              isShellNavigator: true,
+              clipBehavior: _clipBehaviorFor(match.route, navigatorKey),
+            );
+            // The Builder gives the PopScope a context below the
+            // ActiveBranchScope of the branch this Navigator belongs to, so
+            // that canPop is kept up to date when the active branch changes.
+            return Builder(
+              builder: (BuildContext context) {
+                return PopScope(
+                  // Prevent ShellRoute from being popped, for example
+                  // by an iOS back gesture, when the route has active
+                  // sub-routes.
+                  //
+                  // Inactive StatefulShellRoute branches stay mounted, and
+                  // their PopScope is registered on the same enclosing route
+                  // as the one of the active branch. Since a single doNotPop
+                  // vote suppresses the pop for all of them, only the active
+                  // branch may hold back a pop.
+                  // TODO(LukasMirbt): Remove when minimum flutter version includes
+                  // https://github.com/flutter/flutter/pull/152330.
+                  canPop: match.matches.length == 1 || !ActiveBranchScope.isActiveBranchOf(context),
+                  child: navigator,
+                );
+              },
             );
           },
     );

packages/go_router/lib/src/misc/active_branch_scope.dart

@@ -0,0 +1,36 @@
+// Copyright 2013 The Flutter Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/widgets.dart';
+import 'package:meta/meta.dart';
+
+/// Marks whether the subtree below it belongs to the currently active branch
+/// of a `StatefulShellRoute`.
+///
+/// Every branch of a `StatefulShellRoute` stays mounted, including the
+/// inactive ones. Widgets that must only take effect for the branch the user
+/// is currently looking at can use [isActiveBranchOf] to find out whether they
+/// are part of the active branch.
+@internal
+class ActiveBranchScope extends InheritedWidget {
+  /// Constructs an [ActiveBranchScope].
+  const ActiveBranchScope({required this.isActive, required super.child, super.key});
+
+  /// Whether the subtree below this widget belongs to the active branch of the
+  /// enclosing `StatefulShellRoute`.
+  final bool isActive;
+
+  /// Whether [context] is part of the active branch of the closest enclosing
+  /// `StatefulShellRoute`.
+  ///
+  /// Returns true when [context] is not below a `StatefulShellRoute`, for
+  /// instance for the [Navigator] of a plain `ShellRoute`.
+  ///
+  /// The calling widget is rebuilt whenever the active branch changes.
+  static bool isActiveBranchOf(BuildContext context) =>
+      context.dependOnInheritedWidgetOfExactType<ActiveBranchScope>()?.isActive ?? true;
+
+  @override
+  bool updateShouldNotify(ActiveBranchScope oldWidget) => isActive != oldWidget.isActive;
+}

packages/go_router/lib/src/route.dart

@@ -15,6 +15,7 @@ import 'package:meta/meta.dart' as meta;
 
 import 'configuration.dart';
 import 'match.dart';
+import 'misc/active_branch_scope.dart';
 import 'path_utils.dart';
 import 'router.dart';
 import 'state.dart';
@@ -1635,13 +1636,17 @@ class StatefulNavigationShellState extends State<StatefulNavigationShell> with R
 
   @override
   Widget build(BuildContext context) {
+    // A nested StatefulShellRoute is only active if the branch hosting it is
+    // active as well.
+    final bool isActiveShell = ActiveBranchScope.isActiveBranchOf(context);
     final List<Widget> children = route.branches
-        .map(
-          (StatefulShellBranch branch) => _BranchNavigatorProxy(
+        .mapIndexed(
+          (int index, StatefulShellBranch branch) => _BranchNavigatorProxy(
             key: ValueKey<GlobalKey<NavigatorState>>(branch.navigatorKey),
             branch: branch,
             navigatorForBranch: (StatefulShellBranch branch) =>
                 _branchState[branch.navigatorKey]?.navigator,
+            isActive: isActiveShell && index == currentIndex,
           ),
         )
         .toList();
@@ -1712,11 +1717,19 @@ typedef _NavigatorForBranch = Widget? Function(StatefulShellBranch);
 /// important for container implementations that cache child widgets,
 /// such as [TabBarView].
 class _BranchNavigatorProxy extends StatefulWidget {
-  const _BranchNavigatorProxy({super.key, required this.branch, required this.navigatorForBranch});
+  const _BranchNavigatorProxy({
+    super.key,
+    required this.branch,
+    required this.navigatorForBranch,
+    required this.isActive,
+  });
 
   final StatefulShellBranch branch;
   final _NavigatorForBranch navigatorForBranch;
 
+  /// Whether [branch] is the currently active branch.
+  final bool isActive;
+
   @override
   State<StatefulWidget> createState() => _BranchNavigatorProxyState();
 }
@@ -1729,7 +1742,10 @@ class _BranchNavigatorProxyState extends State<_BranchNavigatorProxy>
   @override
   Widget build(BuildContext context) {
     super.build(context);
-    return widget.navigatorForBranch(widget.branch) ?? const SizedBox.shrink();
+    return ActiveBranchScope(
+      isActive: widget.isActive,
+      child: widget.navigatorForBranch(widget.branch) ?? const SizedBox.shrink(),
+    );
   }
 
   @override

packages/go_router/pending_changelogs/fix_issue_193098.yaml

@@ -0,0 +1,3 @@
+changelog: |
+  - Fixes the system back button being ignored after switching away from a `StatefulShellRoute` branch that still has an active sub-route.
+version: patch