fix navbar freezing fix

lib/ui/app_shell.dart

@@ -1,5 +1,6 @@
 import 'package:dynamic_color/dynamic_color.dart';
 import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
 import 'package:go_router/go_router.dart';
 import 'package:mileograph_flutter/components/login/login.dart';
 import 'package:mileograph_flutter/components/pages/calculator.dart';
@@ -17,6 +18,28 @@ import 'package:mileograph_flutter/services/data_service.dart';
 import 'package:mileograph_flutter/services/navigation_guard.dart';
 import 'package:provider/provider.dart';
 
+final GlobalKey<NavigatorState> _shellNavigatorKey = GlobalKey<NavigatorState>();
+
+const List<String> _contentPages = [
+  "/",
+  "/calculator",
+  "/legs",
+  "/traction",
+  "/trips",
+  "/add",
+];
+
+const int _addTabIndex = 5;
+
+int tabIndexForPath(String path) {
+  final newIndex = _contentPages.indexWhere((routePath) {
+    if (path == routePath) return true;
+    if (routePath == '/') return path == '/';
+    return path.startsWith('$routePath/');
+  });
+  return newIndex < 0 ? 0 : newIndex;
+}
+
 class MyApp extends StatefulWidget {
   const MyApp({super.key});
 
@@ -52,6 +75,7 @@ class _MyAppState extends State<MyApp> {
       },
       routes: [
         ShellRoute(
+          navigatorKey: _shellNavigatorKey,
           builder: (context, state, child) => MyHomePage(child: child),
           routes: [
             GoRoute(path: '/', builder: (context, state) => const Dashboard()),
@@ -153,26 +177,7 @@ class MyHomePage extends StatefulWidget {
 }
 
 class _MyHomePageState extends State<MyHomePage> {
-  final List<String> contentPages = [
-    "/",
-    "/calculator",
-    "/legs",
-    "/traction",
-    "/trips",
-    "/add",
-  ];
-
-  int _getIndexFromLocation(String location) {
-    int newIndex = contentPages.indexWhere((path) {
-      if (location == path) return true;
-      if (path == '/') return location == '/';
-      return location.startsWith('$path/');
-    });
-    if (newIndex < 0) {
-      return 0;
-    }
-    return newIndex;
-  }
+  List<String> get contentPages => _contentPages;
 
   Future<void> _onItemTapped(int index, int currentIndex) async {
     if (index < 0 || index >= contentPages.length || index == currentIndex) {
@@ -184,6 +189,10 @@ class _MyHomePageState extends State<MyHomePage> {
     });
   }
 
+  int? _lastTabIndex;
+  final List<int> _tabHistory = [];
+  bool _handlingBackNavigation = false;
+
   bool _fetched = false;
 
   @override
@@ -221,8 +230,12 @@ class _MyHomePageState extends State<MyHomePage> {
 
   @override
   Widget build(BuildContext context) {
-    final location = GoRouterState.of(context).uri.toString();
-    final pageIndex = _getIndexFromLocation(location);
+    final uri = GoRouterState.of(context).uri;
+    final pageIndex = tabIndexForPath(uri.path);
+    _recordTabChange(pageIndex);
+    if (pageIndex != _addTabIndex) {
+      NavigationGuard.unregister();
+    }
     final homepageReady = context.select<DataService, bool>(
       (data) => data.homepageStats != null || !data.isHomepageLoading,
     );
@@ -232,42 +245,90 @@ class _MyHomePageState extends State<MyHomePage> {
         ? widget.child
         : const Center(child: CircularProgressIndicator());
 
-    return Scaffold(
-      appBar: AppBar(
-        backgroundColor: Theme.of(context).colorScheme.inversePrimary,
-        title: Text.rich(
-          TextSpan(
-            children: const [
-              TextSpan(text: "Mile"),
-              TextSpan(text: "O", style: TextStyle(color: Colors.red)),
-              TextSpan(text: "graph"),
-            ],
-            style: const TextStyle(
-              decoration: TextDecoration.none,
-              color: Colors.white,
-              fontFamily: "Tomatoes",
+    return PopScope(
+      canPop: false,
+      onPopInvokedWithResult: (didPop, _) async {
+        if (didPop) return;
+
+        final shellNav = _shellNavigatorKey.currentState;
+        if (shellNav != null && shellNav.canPop()) {
+          shellNav.pop();
+          return;
+        }
+
+        if (_tabHistory.isNotEmpty) {
+          final previousTab = _tabHistory.removeLast();
+          if (!mounted) return;
+          _handlingBackNavigation = true;
+          context.go(contentPages[previousTab]);
+          return;
+        }
+
+        if (pageIndex != 0) {
+          if (!mounted) return;
+          _handlingBackNavigation = true;
+          context.go(contentPages[0]);
+          return;
+        }
+
+        SystemNavigator.pop();
+      },
+      child: Scaffold(
+        appBar: AppBar(
+          backgroundColor: Theme.of(context).colorScheme.inversePrimary,
+          title: Text.rich(
+            TextSpan(
+              children: const [
+                TextSpan(text: "Mile"),
+                TextSpan(text: "O", style: TextStyle(color: Colors.red)),
+                TextSpan(text: "graph"),
+              ],
+              style: const TextStyle(
+                decoration: TextDecoration.none,
+                color: Colors.white,
+                fontFamily: "Tomatoes",
+              ),
             ),
           ),
+          actions: [
+            const IconButton(onPressed: null, icon: Icon(Icons.account_circle)),
+            IconButton(onPressed: auth.logout, icon: const Icon(Icons.logout)),
+          ],
         ),
-        actions: [
-          const IconButton(onPressed: null, icon: Icon(Icons.account_circle)),
-          IconButton(onPressed: auth.logout, icon: const Icon(Icons.logout)),
-        ],
+        bottomNavigationBar: NavigationBar(
+          selectedIndex: pageIndex,
+          onDestinationSelected: (int index) => _onItemTapped(index, pageIndex),
+          destinations: const [
+            NavigationDestination(icon: Icon(Icons.home), label: "Home"),
+            NavigationDestination(icon: Icon(Icons.route), label: "Calculator"),
+            NavigationDestination(icon: Icon(Icons.list), label: "Entries"),
+            NavigationDestination(icon: Icon(Icons.train), label: "Traction"),
+            NavigationDestination(icon: Icon(Icons.book), label: "Trips"),
+            NavigationDestination(icon: Icon(Icons.add), label: "Add"),
+          ],
+        ),
+        body: currentPage,
       ),
-      bottomNavigationBar: NavigationBar(
-        selectedIndex: pageIndex,
-        onDestinationSelected: (int index) => _onItemTapped(index, pageIndex),
-        destinations: const [
-          NavigationDestination(icon: Icon(Icons.home), label: "Home"),
-          NavigationDestination(icon: Icon(Icons.route), label: "Calculator"),
-          NavigationDestination(icon: Icon(Icons.list), label: "Entries"),
-          NavigationDestination(icon: Icon(Icons.train), label: "Traction"),
-          NavigationDestination(icon: Icon(Icons.book), label: "Trips"),
-          NavigationDestination(icon: Icon(Icons.add), label: "Add"),
-        ],
-      ),
-      body: currentPage,
     );
   }
-}
 
+  void _recordTabChange(int pageIndex) {
+    final last = _lastTabIndex;
+    if (last == null) {
+      _lastTabIndex = pageIndex;
+      return;
+    }
+    if (last == pageIndex) return;
+
+    if (_handlingBackNavigation) {
+      _handlingBackNavigation = false;
+      _lastTabIndex = pageIndex;
+      return;
+    }
+
+    if (_tabHistory.isEmpty || _tabHistory.last != last) {
+      _tabHistory.add(last);
+    }
+    _lastTabIndex = pageIndex;
+  }
+}