Fix DropdownMenu menu is detached from the text field (#159665)

## Description

This PR fixes `DropdownMenu` menu vertical position when tight
constraints are given to the `DropdownMenu`.

Before:


![image](https://github.com/user-attachments/assets/d8d9b857-07ed-4d3c-afe2-44518d979e66)

After:


![image](https://github.com/user-attachments/assets/a71ea3c8-a408-43a1-8c99-c2612ad4b24b)



## Related Issue

Fixes [DropdownMenu does not correctly handle incoming maxWidth and
minHeight
constraints](https://github.com/flutter/flutter/issues/147076).
(It fixes the minHeight part, the maxWidth part was already fixed in
https://github.com/flutter/flutter/pull/147233).

## Tests

Adds 4 tests.

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

@@ -1022,6 +1022,17 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
       );
     }
 
+    // Wrap the menu anchor with an Align to narrow down the constraints.
+    // Without this Align, when tight constraints are applied to DropdownMenu,
+    // the menu will appear below these constraints instead of below the
+    // text field.
+    menuAnchor = Align(
+      alignment: AlignmentDirectional.topStart,
+      widthFactor: 1.0,
+      heightFactor: 1.0,
+      child: menuAnchor,
+    );
+
     return Actions(
       actions: <Type, Action<Intent>>{
         _ArrowUpIntent: CallbackAction<_ArrowUpIntent>(

packages/flutter/test/material/dropdown_menu_test.dart

@@ -3488,6 +3488,94 @@ void main() {
     // None of the menus should be closed.
     expect(dropdownMenuAnchor.controller!.isOpen, true);
   });
+
+  group('The menu is attached at the bottom of the TextField', () {
+    // Define the expected text field bottom instead of querying it using
+    // tester.getRect because when tight constraints are applied to the
+    // Dropdown the TextField bounds are expanded while the visible size
+    // remains 56 pixels.
+    const double textFieldBottom = 56.0;
+
+    testWidgets('when given loose constraints and expandedInsets is set', (WidgetTester tester) async {
+      await tester.pumpWidget(MaterialApp(
+        home: Scaffold(
+          body: DropdownMenu<TestMenu>(
+            expandedInsets: EdgeInsets.zero,
+            initialSelection: TestMenu.mainMenu3,
+            dropdownMenuEntries: menuChildrenWithIcons,
+          ),
+        ),
+      ));
+
+      // Open the menu.
+      await tester.tap(find.byType(TextField));
+      await tester.pump();
+
+      expect(tester.getRect(findMenuMaterial()).top, textFieldBottom);
+    });
+
+    testWidgets('when given tight constraints and expandedInsets is set', (WidgetTester tester) async {
+      await tester.pumpWidget(MaterialApp(
+        home: Scaffold(
+          body: SizedBox(
+            width: 200,
+            height: 300,
+            child: DropdownMenu<TestMenu>(
+              expandedInsets: EdgeInsets.zero,
+              initialSelection: TestMenu.mainMenu3,
+              dropdownMenuEntries: menuChildrenWithIcons,
+            ),
+          ),
+        ),
+      ));
+
+      // Open the menu.
+      await tester.tap(find.byType(TextField));
+      await tester.pump();
+
+      expect(tester.getRect(findMenuMaterial()).top, textFieldBottom);
+    });
+
+    // Regression test for https://github.com/flutter/flutter/issues/147076.
+    testWidgets('when given loose constraints and expandedInsets is not set', (WidgetTester tester) async {
+      await tester.pumpWidget(MaterialApp(
+        home: Scaffold(
+          body: DropdownMenu<TestMenu>(
+            initialSelection: TestMenu.mainMenu3,
+            dropdownMenuEntries: menuChildrenWithIcons,
+          ),
+        ),
+      ));
+
+      // Open the menu.
+      await tester.tap(find.byType(TextField));
+      await tester.pump();
+
+      expect(tester.getRect(findMenuMaterial()).top, textFieldBottom);
+    });
+
+    // Regression test for https://github.com/flutter/flutter/issues/147076.
+    testWidgets('when given tight constraints and expandedInsets is not set', (WidgetTester tester) async {
+      await tester.pumpWidget(MaterialApp(
+        home: Scaffold(
+          body: SizedBox(
+            width: 200,
+            height: 300,
+            child: DropdownMenu<TestMenu>(
+              initialSelection: TestMenu.mainMenu3,
+              dropdownMenuEntries: menuChildrenWithIcons,
+            ),
+          ),
+        ),
+      ));
+
+      // Open the menu.
+      await tester.tap(find.byType(TextField));
+      await tester.pump();
+
+      expect(tester.getRect(findMenuMaterial()).top, textFieldBottom);
+    });
+  });
 }
 
 enum TestMenu {