add a test for popupMenuButton (#80930)

packages/flutter/test/material/popup_menu_test.dart

@@ -2209,6 +2209,71 @@ void main() {
     await tester.pumpAndSettle();
     expect(find.text('foo'), findsOneWidget);
   });
+
+  // Regression test for https://github.com/flutter/flutter/issues/80869
+  testWidgets('The menu position test in the scrollable widget', (WidgetTester tester) async {
+    final GlobalKey buttonKey = GlobalKey();
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: SingleChildScrollView(
+            child: Column(
+              children: <Widget>[
+                const SizedBox(height: 100),
+                PopupMenuButton<int>(
+                  child: SizedBox(
+                    key: buttonKey,
+                    height: 10.0,
+                    width: 10.0,
+                    child: const ColoredBox(
+                      color: Colors.pink,
+                    ),
+                  ),
+                  itemBuilder: (BuildContext context) => <PopupMenuEntry<int>>[
+                    const PopupMenuItem<int>(child: Text('-1-'), value: 1,),
+                    const PopupMenuItem<int>(child: Text('-2-'), value: 2,),
+                  ],
+                ),
+                const SizedBox(height: 600),
+              ],
+            ),
+          ),
+        ),
+      ),
+    );
+
+    // Open the menu.
+    await tester.tap(find.byKey(buttonKey));
+    await tester.pumpAndSettle();
+
+    Offset button = tester.getTopLeft(find.byKey(buttonKey));
+    expect(button, const Offset(0.0, 100.0));
+
+    Offset popupMenu = tester.getTopLeft(find.byType(SingleChildScrollView).last);
+    // The menu should be positioned directly next to the top of the button.
+    // The 8.0 pixels is [_kMenuScreenPadding].
+    expect(popupMenu, const Offset(8.0, 100.0));
+
+    // Close the menu.
+    await tester.tap(find.byKey(buttonKey), warnIfMissed: false);
+    await tester.pumpAndSettle();
+
+    // Scroll a little bit.
+    await tester.drag(find.byType(SingleChildScrollView), const Offset(0.0, -50.0));
+
+    button = tester.getTopLeft(find.byKey(buttonKey));
+    expect(button, const Offset(0.0, 50.0));
+
+    // Open the menu again.
+    await tester.tap(find.byKey(buttonKey));
+    await tester.pumpAndSettle();
+
+    popupMenu = tester.getTopLeft(find.byType(SingleChildScrollView).last);
+    // The menu should be positioned directly next to the top of the button.
+    // The 8.0 pixels is [_kMenuScreenPadding].
+    expect(popupMenu, const Offset(8.0, 50.0));
+  });
 }
 
 class TestApp extends StatefulWidget {