Add vertical alignment offset to the ```MenuAnchor``` widget when overflowing (#123740)

Positioning of cascading menus.

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

@@ -3177,7 +3177,12 @@ class _MenuLayout extends SingleChildLayoutDelegate {
       } else if (offBottom(y)) {
         final double newY = anchorRect.top - childSize.height;
         if (!offTop(newY)) {
-          y = newY;
+          // Only move the menu up if its parent is horizontal (MenuAchor/MenuBar).
+          if (parentOrientation == Axis.horizontal) {
+            y = newY - alignmentOffset.dy;
+          } else {
+            y = newY;
+          }
         } else {
           y = allowedRect.bottom - childSize.height;
         }

packages/flutter/test/material/menu_anchor_test.dart

@@ -2481,6 +2481,105 @@ void main() {
       );
     });
 
+    testWidgets('vertically constrained menus are positioned above the anchor by default', (WidgetTester tester) async {
+      await changeSurfaceSize(tester, const Size(800, 600));
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Builder(
+            builder: (BuildContext context) {
+              return Directionality(
+                textDirection: TextDirection.ltr,
+                child: Align(
+                  alignment: Alignment.bottomLeft,
+                  child: MenuAnchor(
+                    menuChildren: const <Widget> [
+                      MenuItemButton(child: Text('Button1'),
+                      ),
+                    ],
+                    builder: (BuildContext context, MenuController controller, Widget? child) {
+                      return FilledButton(
+                        onPressed: () {
+                          if (controller.isOpen) {
+                            controller.close();
+                          } else {
+                            controller.open();
+                          }
+                        },
+                        child: const Text('Tap me'),
+                      );
+                    },
+                  ),
+                ),
+              );
+            },
+          ),
+        ),
+      );
+
+      await tester.pump();
+      await tester.tap(find.text('Tap me'));
+      await tester.pump();
+
+      expect(find.byType(MenuItemButton), findsNWidgets(1));
+      // Test the default offset (0, 0) vertical position.
+      expect(
+        collectSubmenuRects(),
+        equals(const <Rect>[
+          Rect.fromLTRB(0.0, 488.0, 122.0, 552.0),
+        ]),
+      );
+    });
+
+    testWidgets('vertically constrained menus are positioned above the anchor with the provided offset', (WidgetTester tester) async {
+      await changeSurfaceSize(tester, const Size(800, 600));
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Builder(
+            builder: (BuildContext context) {
+              return Directionality(
+                textDirection: TextDirection.ltr,
+                child: Align(
+                  alignment: Alignment.bottomLeft,
+                  child: MenuAnchor(
+                    alignmentOffset: const Offset(0, 50),
+                    menuChildren: const <Widget> [
+                      MenuItemButton(child: Text('Button1'),
+                      ),
+                    ],
+                    builder: (BuildContext context, MenuController controller, Widget? child) {
+                      return FilledButton(
+                        onPressed: () {
+                          if (controller.isOpen) {
+                            controller.close();
+                          } else {
+                            controller.open();
+                          }
+                        },
+                        child: const Text('Tap me'),
+                      );
+                    },
+                  ),
+                ),
+              );
+            },
+          ),
+        ),
+      );
+
+      await tester.pump();
+      await tester.tap(find.text('Tap me'));
+      await tester.pump();
+
+      expect(find.byType(MenuItemButton), findsNWidgets(1));
+      // Test the offset (0, 50) vertical position.
+      expect(
+        collectSubmenuRects(),
+        equals(const <Rect>[
+          Rect.fromLTRB(0.0, 438.0, 122.0, 502.0),
+        ]),
+      );
+    });
+
     Future<void> buildDensityPaddingApp(WidgetTester tester, {
       required TextDirection textDirection,
       VisualDensity visualDensity = VisualDensity.standard,