Check to see if previous page is laid out before starting hero flight (#169633)

Fixes #168267

When going back to a page that never rendered with a hero transition,
the app was hanging. This adds a check to see if a page needs to layout
before starting the hero flight, and if so, adding a frame delay.

## Pre-launch Checklist

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we expect every widget to implement].
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- [x] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
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Fixes] where supported.
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Author: MitchellGoodwin

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

@@ -677,12 +677,11 @@ class _HeroFlight {
       final HeroFlightDirection type = initialManifest.type;
       switch (type) {
         case HeroFlightDirection.pop:
-          return initial.value == 1.0 && initialManifest.isUserGestureTransition
+          return initialManifest.isUserGestureTransition
               // During user gesture transitions, the animation controller isn't
-              // driving the reverse transition, but should still be in a previously
-              // completed stage with the initial value at 1.0.
-              ? initial.status == AnimationStatus.completed
-              : initial.status == AnimationStatus.reverse;
+              // driving the reverse transition, so the status is not important.
+              ||
+              initial.status == AnimationStatus.reverse;
         case HeroFlightDirection.push:
           return initial.value == 0.0 && initial.status == AnimationStatus.forward;
       }
@@ -923,8 +922,15 @@ class HeroController extends NavigatorObserver {
 
     // For pop transitions driven by a user gesture: if the "to" page has
     // maintainState = true, then the hero's final dimensions can be measured
-    // immediately because their page's layout is still valid.
-    if (isUserGestureTransition && flightType == HeroFlightDirection.pop && toRoute.maintainState) {
+    // immediately because their page's layout is still valid. Unless due to directly
+    // adding routes to the pages stack causing the route to never get laid out.
+    final RenderBox? fromRouteRenderBox = toRoute.subtreeContext?.findRenderObject() as RenderBox?;
+    final bool hasValidSize =
+        (fromRouteRenderBox?.hasSize ?? false) && fromRouteRenderBox!.size.isFinite;
+    if (isUserGestureTransition &&
+        flightType == HeroFlightDirection.pop &&
+        toRoute.maintainState &&
+        hasValidSize) {
       _startHeroTransition(fromRoute, toRoute, flightType, isUserGestureTransition);
     } else {
       // Otherwise, delay measuring until the end of the next frame to allow
@@ -934,7 +940,6 @@ class HeroController extends NavigatorObserver {
       // frame completes, we'll know where the heroes in the `to` route are
       // going to end up, and the `to` route will go back onstage.
       toRoute.offstage = toRoute.animation!.value == 0.0;
-
       WidgetsBinding.instance.addPostFrameCallback((Duration value) {
         if (fromRoute.navigator == null || toRoute.navigator == null) {
           return;

packages/flutter/test/widgets/heroes_test.dart

@@ -216,6 +216,73 @@ class MyStatefulWidgetState extends State<MyStatefulWidget> {
   Widget build(BuildContext context) => Text(widget.value);
 }
 
+class DeepLinkApp extends StatefulWidget {
+  const DeepLinkApp({super.key});
+
+  static const CupertinoPage<void> _homeScreen = CupertinoPage<void>(
+    name: '/',
+    child: CupertinoPageScaffold(
+      navigationBar: CupertinoNavigationBar(middle: Text('First')),
+      child: Center(child: Text('Home Screen')),
+    ),
+  );
+  static const CupertinoPage<void> _middleScreen = CupertinoPage<void>(
+    name: '/middle',
+    child: CupertinoPageScaffold(
+      navigationBar: CupertinoNavigationBar(middle: Text('Second')),
+      child: Center(child: Text('Middle Screen')),
+    ),
+  );
+  static const CupertinoPage<void> _lastScreen = CupertinoPage<void>(
+    name: '/middle/last',
+    child: CupertinoPageScaffold(
+      navigationBar: CupertinoNavigationBar(middle: Text('Third')),
+      child: Center(child: Text('Last Screen')),
+    ),
+  );
+
+  @override
+  DeepLinkAppState createState() => DeepLinkAppState();
+}
+
+class DeepLinkAppState extends State<DeepLinkApp> {
+  late List<Page<dynamic>> _pages;
+
+  @override
+  void initState() {
+    super.initState();
+    _pages = <Page<dynamic>>[DeepLinkApp._homeScreen];
+  }
+
+  void goToDeepScreen() {
+    setState(() {
+      _pages = <Page<dynamic>>[
+        DeepLinkApp._homeScreen,
+        DeepLinkApp._middleScreen,
+        DeepLinkApp._lastScreen,
+      ];
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return CupertinoApp(
+      builder: (BuildContext context, Widget? child) {
+        return Navigator(
+          pages: _pages,
+          onDidRemovePage: (Page<Object?> page) {
+            setState(() {
+              if (_pages.length > 1) {
+                _pages = List<Page<dynamic>>.from(_pages)..removeLast();
+              }
+            });
+          },
+        );
+      },
+    );
+  }
+}
+
 Future<void> main() async {
   final ui.Image testImage = await createTestImage();
 
@@ -2984,6 +3051,41 @@ Future<void> main() async {
     }),
   );
 
+  // Regression test for https://github.com/flutter/flutter/issues/168267.
+  testWidgets('Check if previous page is laid out on backswipe gesture before flight', (
+    WidgetTester tester,
+  ) async {
+    final GlobalKey<DeepLinkAppState> appKey = GlobalKey();
+    await tester.pumpWidget(DeepLinkApp(key: appKey));
+
+    await tester.pumpAndSettle();
+
+    expect(find.text('Home Screen'), findsOneWidget);
+    expect(find.text('Last Screen'), findsNothing);
+
+    appKey.currentState?.goToDeepScreen();
+
+    await tester.pumpAndSettle();
+
+    expect(find.text('Home Screen'), findsNothing);
+    expect(find.text('Last Screen'), findsOneWidget);
+
+    final TestGesture gesture = await tester.startGesture(const Offset(0.01, 300));
+
+    await gesture.moveTo(const Offset(10, 300));
+    await tester.pump();
+    // Should not throw an assert here for size and finite space.
+    await gesture.moveTo(const Offset(500, 300));
+    await tester.pump();
+
+    await gesture.up();
+    await tester.pumpAndSettle();
+
+    expect(find.text('Home Screen'), findsNothing);
+    expect(find.text('Middle Screen'), findsOneWidget);
+    expect(find.text('Last Screen'), findsNothing);
+  });
+
   // Regression test for https://github.com/flutter/flutter/issues/40239.
   testWidgets(
     'In a pop transition, when fromHero is null, the to hero should eventually become visible',