Fix canpop logic to be more robust (#73993)

packages/flutter/lib/src/material/app_bar.dart

@@ -709,8 +709,6 @@ class _AppBarState extends State<AppBar> {
     Scaffold.of(context).openEndDrawer();
   }
 
-  bool? hadBackButtonWhenRouteWasActive;
-
   @override
   Widget build(BuildContext context) {
     assert(!widget.primary || debugCheckHasMediaQuery(context));
@@ -723,11 +721,7 @@ class _AppBarState extends State<AppBar> {
 
     final bool hasDrawer = scaffold?.hasDrawer ?? false;
     final bool hasEndDrawer = scaffold?.hasEndDrawer ?? false;
-    hadBackButtonWhenRouteWasActive ??= false;
-    if (parentRoute?.isActive == true) {
-      hadBackButtonWhenRouteWasActive = parentRoute!.canPop;
-    }
-    assert(hadBackButtonWhenRouteWasActive != null);
+    final bool canPop = parentRoute?.canPop ?? false;
     final bool useCloseButton = parentRoute is PageRoute<dynamic> && parentRoute.fullscreenDialog;
 
     final double toolbarHeight = widget.toolbarHeight ?? kToolbarHeight;
@@ -796,7 +790,7 @@ class _AppBarState extends State<AppBar> {
           tooltip: MaterialLocalizations.of(context).openAppDrawerTooltip,
         );
       } else {
-        if (!hasEndDrawer && hadBackButtonWhenRouteWasActive!)
+        if (!hasEndDrawer && canPop)
           leading = useCloseButton ? const CloseButton() : const BackButton();
       }
     }

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

@@ -464,10 +464,7 @@ abstract class Route<T> {
     return currentRouteEntry.route == this;
   }
 
-  /// Whether this route is the bottom-most route on the navigator.
-  ///
-  /// If this is true, then [Navigator.canPop] will return false if this route's
-  /// [willHandlePopInternally] returns false.
+  /// Whether this route is the bottom-most active route on the navigator.
   ///
   /// If [isFirst] and [isCurrent] are both true then this is the only route on
   /// the navigator (and [isActive] will also be true).
@@ -483,6 +480,20 @@ abstract class Route<T> {
     return currentRouteEntry.route == this;
   }
 
+  /// Whether there is at least one active route underneath this route.
+  @protected
+  bool get hasActiveRouteBelow {
+    if (_navigator == null)
+      return false;
+    for (final _RouteEntry entry in _navigator!._history) {
+      if (entry.route == this)
+        return false;
+      if (_RouteEntry.isPresentPredicate(entry))
+        return true;
+    }
+    return false;
+  }
+
   /// Whether this route is on the navigator.
   ///
   /// If the route is not only active, but also the current route (the top-most

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

@@ -1483,10 +1483,13 @@ abstract class ModalRoute<T> extends TransitionRoute<T> with LocalHistoryRoute<T
 
   /// Whether this route can be popped.
   ///
+  /// A route can be popped if there is at least one active route below it, or
+  /// if [willHandlePopInternally] returns true.
+  ///
   /// When this changes, if the route is visible, the route will
   /// rebuild, and any widgets that used [ModalRoute.of] will be
   /// notified.
-  bool get canPop => isActive && (!isFirst || willHandlePopInternally);
+  bool get canPop => hasActiveRouteBelow || willHandlePopInternally;
 
   // Internals
 

packages/flutter/test/cupertino/nav_bar_test.dart

@@ -93,6 +93,36 @@ void main() {
     expect(find.byType(BackdropFilter), findsOneWidget);
   });
 
+  testWidgets('Nav bar displays correctly', (WidgetTester tester) async {
+    final GlobalKey<NavigatorState> navigator = GlobalKey<NavigatorState>();
+    await tester.pumpWidget(
+      CupertinoApp(
+        navigatorKey: navigator,
+        home: const CupertinoNavigationBar(
+          middle: Text('Page 1'),
+        ),
+      ),
+    );
+    navigator.currentState!.push<void>(CupertinoPageRoute<void>(
+        builder: (BuildContext context) {
+          return const CupertinoNavigationBar(
+            middle: Text('Page 2'),
+          );
+        }
+    ));
+    await tester.pumpAndSettle();
+    expect(find.byType(CupertinoNavigationBarBackButton), findsOneWidget);
+    // Pops the page 2
+    navigator.currentState!.pop();
+    await tester.pump();
+    // Needs another pump to trigger the rebuild;
+    await tester.pump();
+    // The back button should still persist;
+    expect(find.byType(CupertinoNavigationBarBackButton), findsOneWidget);
+    // The app does not crash
+    expect(tester.takeException(), isNull);
+  });
+
   testWidgets('Can specify custom padding', (WidgetTester tester) async {
     final Key middleBox = GlobalKey();
     await tester.pumpWidget(