The initial/selected item on popup menu should always be visible (#143118)

Fixes #142895

With the change of #143121, this PR is to add auto scroll to `PopupMenuButton` so when we open the menu, it will automatically scroll to the selected item.

https://github.com/flutter/flutter/assets/36861262/c2bc0395-0641-4e7a-a54d-57a8e62ee26f

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

@@ -4,6 +4,7 @@
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
+import 'package:flutter/scheduler.dart';
 import 'package:flutter/widgets.dart';
 
 import 'color_scheme.dart';
@@ -570,12 +571,14 @@ class _CheckedPopupMenuItemState<T> extends PopupMenuItemState<T, CheckedPopupMe
 class _PopupMenu<T> extends StatelessWidget {
   const _PopupMenu({
     super.key,
+    required this.itemKeys,
     required this.route,
     required this.semanticLabel,
     this.constraints,
     required this.clipBehavior,
   });
 
+  final List<GlobalKey> itemKeys;
   final _PopupMenuRoute<T> route;
   final String? semanticLabel;
   final BoxConstraints? constraints;
@@ -609,6 +612,7 @@ class _PopupMenu<T> extends StatelessWidget {
             route.itemSizes[i] = size;
           },
           child: FadeTransition(
+            key: itemKeys[i],
             opacity: opacity,
             child: item,
           ),
@@ -791,6 +795,7 @@ class _PopupMenuRoute<T> extends PopupRoute<T> {
   _PopupMenuRoute({
     required this.position,
     required this.items,
+    required this.itemKeys,
     this.initialValue,
     this.elevation,
     this.surfaceTintColor,
@@ -811,6 +816,7 @@ class _PopupMenuRoute<T> extends PopupRoute<T> {
 
   final RelativeRect position;
   final List<PopupMenuEntry<T>> items;
+  final List<GlobalKey> itemKeys;
   final List<Size?> itemSizes;
   final T? initialValue;
   final double? elevation;
@@ -836,6 +842,14 @@ class _PopupMenuRoute<T> extends PopupRoute<T> {
     return super.createAnimation();
   }
 
+  void scrollTo(int selectedItemIndex) {
+    SchedulerBinding.instance.addPostFrameCallback((_) {
+      if (itemKeys[selectedItemIndex].currentContext != null) {
+        Scrollable.ensureVisible(itemKeys[selectedItemIndex].currentContext!);
+      }
+    });
+  }
+
   @override
   Duration get transitionDuration => popUpAnimationStyle?.duration ?? _kMenuDuration;
 
@@ -859,9 +873,13 @@ class _PopupMenuRoute<T> extends PopupRoute<T> {
         }
       }
     }
+    if (selectedItemIndex != null) {
+      scrollTo(selectedItemIndex);
+    }
 
     final Widget menu = _PopupMenu<T>(
       route: this,
+      itemKeys: itemKeys,
       semanticLabel: semanticLabel,
       constraints: constraints,
       clipBehavior: clipBehavior,
@@ -985,10 +1003,12 @@ Future<T?> showMenu<T>({
       semanticLabel ??= MaterialLocalizations.of(context).popupMenuLabel;
   }
 
+  final List<GlobalKey> menuItemKeys = List<GlobalKey>.generate(items.length, (int index) => GlobalKey());
   final NavigatorState navigator = Navigator.of(context, rootNavigator: useRootNavigator);
   return navigator.push(_PopupMenuRoute<T>(
     position: position,
     items: items,
+    itemKeys: menuItemKeys,
     initialValue: initialValue,
     elevation: elevation,
     shadowColor: shadowColor,

packages/flutter/test/material/popup_menu_test.dart

@@ -3956,6 +3956,59 @@ void main() {
     expect(tester.getSize(find.byType(Material).last), within(distance: 0.1, from: const Size(112.0, 160.0)));
   });
 
+  testWidgets('PopupMenuButton scrolls initial value/selected value to visible', (WidgetTester tester) async {
+    const int length = 50;
+    const int selectedValue = length - 1;
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: Align(
+            alignment: Alignment.bottomCenter,
+            child: PopupMenuButton<int>(
+              itemBuilder: (BuildContext context) {
+                return List<PopupMenuEntry<int>>.generate(length, (int index) {
+                  return PopupMenuItem<int>(value: index, child: Text('item #$index'));
+                });
+              },
+              popUpAnimationStyle: AnimationStyle.noAnimation,
+              initialValue: selectedValue,
+              child: const Text('click here'),
+            ),
+          ),
+        ),
+      ),
+    );
+    await tester.tap(find.text('click here'));
+    await tester.pump();
+
+    // Set up finder and verify basic widget structure.
+    final Finder item49 = find.text('item #49');
+    expect(item49, findsOneWidget);
+
+    // The initially selected menu item should be positioned on screen.
+    final RenderBox initialItem = tester.renderObject<RenderBox>(item49);
+    final Rect initialItemBounds = initialItem.localToGlobal(Offset.zero) & initialItem.size;
+    final Size windowSize = tester.view.physicalSize / tester.view.devicePixelRatio;
+    expect(initialItemBounds.bottomRight.dy, lessThanOrEqualTo(windowSize.height));
+
+    // Select item 20.
+    final Finder item20 = find.text('item #20');
+    await tester.scrollUntilVisible(item20, 500);
+    expect(item20, findsOneWidget);
+    await tester.tap(item20);
+    await tester.pump();
+
+    // Open menu again.
+    await tester.tap(find.text('click here'));
+    await tester.pump();
+    expect(item20, findsOneWidget);
+
+    // The selected menu item should be positioned on screen.
+    final RenderBox selectedItem = tester.renderObject<RenderBox>(item20);
+    final Rect selectedItemBounds = selectedItem.localToGlobal(Offset.zero) & selectedItem.size;
+    expect(selectedItemBounds.bottomRight.dy, lessThanOrEqualTo(windowSize.height));
+  });
+
   testWidgets('PopupMenuButton properly positions a constrained-size popup', (WidgetTester tester) async {
     final Size windowSize = tester.view.physicalSize / tester.view.devicePixelRatio;
     const int length = 50;