Align the Popup Menu under its child (#125534)

Align the `PopupMenu` under its child.

Before:

![image](https://user-images.githubusercontent.com/32538273/234468262-beddb257-5f5a-422f-a678-c26efc52dc07.png)

After:

![image](https://user-images.githubusercontent.com/32538273/234468233-3ea4d741-b04c-4012-bdd7-a36ecedc029c.png)

*List which issues are fixed by this PR. You must list at least one issue.*

Fixes https://github.com/flutter/flutter/issues/125474

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*

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

@@ -1241,12 +1241,16 @@ class PopupMenuButtonState<T> extends State<PopupMenuButton<T>> {
     final RenderBox button = context.findRenderObject()! as RenderBox;
     final RenderBox overlay = Navigator.of(context).overlay!.context.findRenderObject()! as RenderBox;
     final PopupMenuPosition popupMenuPosition = widget.position ?? popupMenuTheme.position ?? PopupMenuPosition.over;
-    final Offset offset;
+    late Offset offset;
     switch (popupMenuPosition) {
       case PopupMenuPosition.over:
         offset = widget.offset;
       case PopupMenuPosition.under:
-        offset = Offset(0.0, button.size.height - (widget.padding.vertical / 2)) + widget.offset;
+        offset = Offset(0.0, button.size.height) + widget.offset;
+        if (widget.child == null) {
+          // Remove the padding of the icon button.
+          offset -= Offset(0.0, widget.padding.vertical / 2);
+        }
     }
     final RelativeRect position = RelativeRect.fromRect(
       Rect.fromPoints(

packages/flutter/test/material/popup_menu_test.dart

@@ -3222,6 +3222,44 @@ void main() {
     await tester.pumpAndSettle();
     expect(currentRouteSetting.name, '/');
   });
+
+  testWidgets('Popup menu is positioned under the child', (WidgetTester tester) async {
+    final Key popupButtonKey = UniqueKey();
+    final Key childKey = UniqueKey();
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Material(
+          child: Column(
+            children: <Widget>[
+              PopupMenuButton<void>(
+                key: popupButtonKey,
+                position: PopupMenuPosition.under,
+                itemBuilder: (BuildContext context) {
+                  return <PopupMenuEntry<void>>[
+                    const PopupMenuItem<void>(
+                      child: Text('Example'),
+                    ),
+                  ];
+                },
+                child: SizedBox(
+                  key: childKey,
+                  height: 50,
+                  width: 50,
+                )
+              ),
+            ],
+          ),
+        ),
+      )
+    );
+
+    await tester.tap(find.byKey(popupButtonKey));
+    await tester.pumpAndSettle();
+
+    final Offset childBottomLeft = tester.getBottomLeft(find.byKey(childKey));
+    final Offset menuTopLeft = tester.getTopLeft(find.bySemanticsLabel('Popup menu'));
+    expect(childBottomLeft, menuTopLeft);
+  });
 }
 
 class TestApp extends StatelessWidget {