fix PopupMenuButton unmounted exception when updating position (#166412)

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This PR fixes an exception thrown when trying to update an unmounted
PopupMenuButton's position, which can happen when a layout change is
triggered during PopupMenuButton's pop animation (see issue's attached
video).
A workaround is to set `popUpAnimationStyle:
AnimationStyle.noAnimation`.
This PR fixes it by returning the last known position if the button is
unmounted.

Exception thrown:` FlutterError (This widget has been unmounted, so the
State no longer has a context (and should be considered defunct).
Consider canceling any active work during "dispose" or using the
"mounted" getter to determine if the State is still active.)`

Code that causes the exception: `final PopupMenuThemeData popupMenuTheme
= PopupMenuTheme.of(context);`

Fixes #163477 

Tested both on stable (3.29.2) and master (a0b1b325341)

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---------

Co-authored-by: Navaron Bracke <brackenavaron@gmail.com>
Co-authored-by: Tong Mu <dkwingsmt@users.noreply.github.com>

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

@@ -1568,7 +1568,17 @@ class PopupMenuButton<T> extends StatefulWidget {
 /// of your button state.
 class PopupMenuButtonState<T> extends State<PopupMenuButton<T>> {
   bool _isMenuExpanded = false;
+  RelativeRect? _lastPosition;
+
   RelativeRect _positionBuilder(BuildContext _, BoxConstraints constraints) {
+    if (!mounted) {
+      // When the route is displayed, the `_positionBuilder` closure is stored.
+      // Even after the button has been unmounted and the context becomes invalid,
+      // the route might keep displaying, and `_positionBuilder` must continue to
+      // work in that case.
+      return _lastPosition ?? RelativeRect.fromSize(Rect.zero, constraints.biggest);
+    }
+
     final PopupMenuThemeData popupMenuTheme = PopupMenuTheme.of(context);
     final RenderBox button = context.findRenderObject()! as RenderBox;
     final RenderBox overlay =
@@ -1598,7 +1608,7 @@ class PopupMenuButtonState<T> extends State<PopupMenuButton<T>> {
       Offset.zero & overlay.size,
     );
 
-    return position;
+    return _lastPosition = position;
   }
 
   /// A method to show a popup menu with the items supplied to

packages/flutter/test/material/popup_menu_test.dart

@@ -2863,6 +2863,71 @@ void main() {
     expect(mediaQueryPadding, EdgeInsets.zero);
   });
 
+  // Regression test for https://github.com/flutter/flutter/issues/163477
+  testWidgets("PopupMenu's overlay can be rebuilt even when the button is unmounted", (
+    WidgetTester tester,
+  ) async {
+    final GlobalKey buttonKey = GlobalKey();
+
+    late StateSetter setState;
+    bool showButton = true;
+
+    Widget widget({required Size viewSize}) {
+      return Center(
+        child: SizedBox(
+          width: viewSize.width,
+          height: viewSize.height,
+          child: MaterialApp(
+            home: Material(
+              child: StatefulBuilder(
+                builder: (BuildContext context, StateSetter innerSetState) {
+                  setState = innerSetState;
+                  return showButton
+                      ? PopupMenuButton<int>(
+                        key: buttonKey,
+                        popUpAnimationStyle: const AnimationStyle(
+                          reverseDuration: Duration(milliseconds: 400),
+                        ),
+                        itemBuilder: (BuildContext context) {
+                          return <PopupMenuEntry<int>>[
+                            PopupMenuItem<int>(value: 1, child: const Text('ACTION'), onTap: () {}),
+                          ];
+                        },
+                      )
+                      : Container();
+                },
+              ),
+            ),
+          ),
+        ),
+      );
+    }
+
+    // Pump a button
+    await tester.pumpWidget(widget(viewSize: const Size(500, 500)));
+
+    // Tap the button to show the menu
+    await tester.tap(find.byKey(buttonKey));
+    await tester.pumpAndSettle();
+    expect(find.text('ACTION'), findsOne);
+    expect(find.byKey(buttonKey), findsOne);
+
+    // Hide the button. The menu still shows since it's placed on a separate route.
+    setState(() {
+      showButton = false;
+    });
+    await tester.pump();
+    expect(find.text('ACTION'), findsOne);
+    expect(find.byKey(buttonKey), findsNothing);
+
+    // Resize the view, causing the menu to rebuild. Before the fix, this
+    // rebuild would lead to a crash, because it relies on context of the button,
+    // which has been unmounted.
+    await tester.pumpWidget(widget(viewSize: const Size(300, 300)));
+
+    expect(tester.takeException(), isNull);
+  });
+
   group('feedback', () {
     late FeedbackTester feedback;