Fix RouterObserver didPop is not called when reverseTransitionDuratio… (#97171)

packages/flutter/lib/src/widgets/navigator.dart

@@ -5023,11 +5023,15 @@ class NavigatorState extends State<Navigator> with TickerProviderStateMixin, Res
     bool? wasDebugLocked;
     assert(() { wasDebugLocked = _debugLocked; _debugLocked = true; return true; }());
     assert(_history.where(_RouteEntry.isRoutePredicate(route)).length == 1);
-    final _RouteEntry entry =  _history.firstWhere(_RouteEntry.isRoutePredicate(route));
-    // For page-based route, the didPop can be called on any life cycle above
-    // pop.
-    assert(entry.currentState == _RouteLifecycle.popping ||
-          (entry.hasPage && entry.currentState.index < _RouteLifecycle.pop.index));
+    final int index = _history.indexWhere(_RouteEntry.isRoutePredicate(route));
+    final _RouteEntry entry =  _history[index];
+    // For page-based route with zero transition, the finalizeRoute can be
+    // called on any life cycle above pop.
+    if (entry.hasPage && entry.currentState.index < _RouteLifecycle.pop.index) {
+      _observedRouteDeletions.add(_NavigatorPopObservation(route, _getRouteBefore(index - 1, _RouteEntry.willBePresentPredicate)?.route));
+    } else {
+      assert(entry.currentState == _RouteLifecycle.popping);
+    }
     entry.finalize();
     // finalizeRoute can be called during _flushHistoryUpdates if a pop
     // finishes synchronously.

packages/flutter/test/widgets/navigator_test.dart

@@ -183,6 +183,47 @@ void main() {
     expect('$exception', startsWith('Navigator operation requested with a context'));
   });
 
+  testWidgets('Zero transition page-based route correctly notifies observers when it is popped', (WidgetTester tester) async {
+    final List<Page<void>> pages = <Page<void>>[
+      const ZeroTransitionPage(name: 'Page 1'),
+      const ZeroTransitionPage(name: 'Page 2'),
+    ];
+    final List<NavigatorObservation> observations = <NavigatorObservation>[];
+
+    final TestObserver observer = TestObserver()
+      ..onPopped = (Route<dynamic>? route, Route<dynamic>? previousRoute) {
+        observations.add(
+          NavigatorObservation(
+            current: route?.settings.name,
+            previous: previousRoute?.settings.name,
+            operation: 'pop',
+          ),
+        );
+      };
+    final GlobalKey<NavigatorState> navigator = GlobalKey<NavigatorState>();
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: Navigator(
+          key: navigator,
+          pages: pages,
+          observers: <NavigatorObserver>[observer],
+          onPopPage: (Route<dynamic> route, dynamic result) {
+            pages.removeLast();
+            return route.didPop(result);
+          },
+        ),
+      ),
+    );
+
+    navigator.currentState!.pop();
+    await tester.pump();
+
+    expect(observations.length, 1);
+    expect(observations[0].current, 'Page 2');
+    expect(observations[0].previous, 'Page 1');
+  });
+
   testWidgets('Navigator.of rootNavigator finds root Navigator', (WidgetTester tester) async {
     await tester.pumpWidget(MaterialApp(
       home: Material(
@@ -3941,6 +3982,22 @@ class AlwaysRemoveTransitionDelegate extends TransitionDelegate<void> {
   }
 }
 
+class ZeroTransitionPage extends Page<void> {
+  const ZeroTransitionPage({
+    LocalKey? key,
+    Object? arguments,
+    required String name,
+  }) : super(key: key, name: name, arguments: arguments);
+
+  @override
+  Route<void> createRoute(BuildContext context) {
+    return NoAnimationPageRoute(
+      settings: this,
+      pageBuilder: (BuildContext context) => Text(name!),
+    );
+  }
+}
+
 class TestPage extends Page<void> {
   const TestPage({
     LocalKey? key,
@@ -3958,15 +4015,17 @@ class TestPage extends Page<void> {
 }
 
 class NoAnimationPageRoute extends PageRouteBuilder<void> {
-  NoAnimationPageRoute({required WidgetBuilder pageBuilder})
-      : super(pageBuilder: (BuildContext context, __, ___) {
+  NoAnimationPageRoute({
+    RouteSettings? settings,
+    required WidgetBuilder pageBuilder
+  }) : super(
+         settings: settings,
+         transitionDuration: Duration.zero,
+         reverseTransitionDuration: Duration.zero,
+         pageBuilder: (BuildContext context, __, ___) {
            return pageBuilder(context);
-        });
-
-  @override
-  AnimationController createAnimationController() {
-    return super.createAnimationController()..value = 1.0;
          }
+       );
 }
 
 class StatefulTestWidget extends StatefulWidget {