Stop DropdownMenu internal scrolling from moving parent Scrollable (#153360)

fixes flutter/flutter#151854
fixes #139113

The regression test uses nested `ListView` because I could not reproduce the issue without using nested `Scrollable`.

The issue is masked when there is only one `Scrollable` outside `DropdownMenu`. While `Scrollable.ensureVisible` can find all the `Scrollable`, the actual scrolling is performed by `ScrollPosition.ensureVisible` , which uses the `RenderObject` tree to find nearest Viewport but it could not find one due to `OverlayPortal` putting the target `RenderObject` at different point. So no scrolling will occur.

However when there are nested `Scrollable`, `Scrollable.ensureVisible` can scroll the outside `Scrollable` normally since no `RenderObject` tree gap exist between the two `Scrollable`

Author: PurplePolyhedron

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

@@ -592,7 +592,7 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
     WidgetsBinding.instance.addPostFrameCallback((_) {
       final BuildContext? highlightContext = buttonItemKeys[currentHighlight!].currentContext;
       if (highlightContext != null) {
-        Scrollable.ensureVisible(highlightContext);
+        Scrollable.of(highlightContext).position.ensureVisible(highlightContext.findRenderObject()!);
       }
     }, debugLabel: 'DropdownMenu.scrollToHighlight');
   }

packages/flutter/test/material/dropdown_menu_test.dart

@@ -2723,6 +2723,36 @@ void main() {
       ),
     );
   });
+
+  // This is a regression test for https://github.com/flutter/flutter/issues/151854.
+  testWidgets('scrollToHighlight does not scroll parent', (WidgetTester tester) async {
+    final ScrollController controller = ScrollController();
+    addTearDown(controller.dispose);
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: ListView(
+            controller: controller,
+            children: <Widget>[
+              ListView(
+                shrinkWrap: true,
+                children: <Widget>[DropdownMenu<TestMenu>(
+                  initialSelection: menuChildren.last.value,
+                  dropdownMenuEntries: menuChildren,
+                )],
+              ),
+              const SizedBox(height: 1000.0),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    await tester.tap(find.byType(TextField).first);
+    await tester.pumpAndSettle();
+    expect(controller.offset, 0.0);
+  });
 }
 
 enum TestMenu {