Fix default minimumSize in dropdownMenu when maximumSize is null (flutter#169438)

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Previous change in `DropDownMenu`
flutter#162380 forced the menu width to
be constrained between the width or anchorWidth and the maximumSize
width menuStyle, but giving a minimum of zero when maximumSize was not
enforced or null. This prevents width or anchorWidth to be used at all
as the minimum will be zero always if maximumSize is null

Now this sets the default value back to width or anchorWidth if
maximumSize is null, keeping consistency of the previous configuration
while using the fix of maximumSize when provided.

Fixes flutter#170970

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---------

Co-authored-by: Qun Cheng <[email protected]>

Author: EdwynZN

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

@@ -1044,7 +1044,7 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
           final double? effectiveMaximumWidth = effectiveMenuStyle!.maximumSize
               ?.resolve(states)
               ?.width;
-          return Size(math.min(widget.width!, effectiveMaximumWidth ?? 0.0), 0.0);
+          return Size(math.min(widget.width!, effectiveMaximumWidth ?? widget.width!), 0.0);
         }),
       );
     } else if (anchorWidth != null) {
@@ -1053,7 +1053,7 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
           final double? effectiveMaximumWidth = effectiveMenuStyle!.maximumSize
               ?.resolve(states)
               ?.width;
-          return Size(math.min(anchorWidth, effectiveMaximumWidth ?? 0.0), 0.0);
+          return Size(math.min(anchorWidth, effectiveMaximumWidth ?? anchorWidth), 0.0);
         }),
       );
     }

packages/flutter/test/material/dropdown_menu_test.dart

@@ -4301,8 +4301,77 @@ void main() {
 
     expect(tester.takeException(), isNull);
     expect(tester.getSize(findMenuItemButton(menuChildren.first.label)).width, 150.0);
+
+    // The overwrite of menuStyle is different when a width is provided but maximumSize is not,
+    // So it needs to be tested separately.
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: DropdownMenu<TestMenu>(
+            width: 200.0,
+            dropdownMenuEntries: menuChildren,
+            menuStyle: const MenuStyle(),
+          ),
+        ),
+      ),
+    );
+
+    await tester.tap(find.byType(TextField));
+    await tester.pumpAndSettle();
+
+    expect(tester.getSize(findMenuItemButton(menuChildren.first.label)).width, 200.0);
   });
 
+  testWidgets(
+    'ensure items are constrained to intrinsic size of DropdownMenu (width or anchor) when no maximumSize',
+    (WidgetTester tester) async {
+      const String shortLabel = 'Male';
+      await tester.pumpWidget(
+        const MaterialApp(
+          home: Scaffold(
+            body: DropdownMenu<int>(
+              width: 200,
+              dropdownMenuEntries: <DropdownMenuEntry<int>>[
+                DropdownMenuEntry<int>(value: 0, label: shortLabel),
+              ],
+              menuStyle: MenuStyle(),
+            ),
+          ),
+        ),
+      );
+
+      await tester.tap(find.byType(TextField));
+      await tester.pumpAndSettle();
+
+      expect(tester.getSize(findMenuItemButton(shortLabel)).width, 200);
+
+      // Use expandedInsets to anchor the TextField to the same size as the parent.
+      await tester.pumpWidget(
+        const MaterialApp(
+          home: Scaffold(
+            body: SizedBox(
+              width: double.infinity,
+              child: DropdownMenu<int>(
+                expandedInsets: EdgeInsets.symmetric(horizontal: 20),
+                dropdownMenuEntries: <DropdownMenuEntry<int>>[
+                  DropdownMenuEntry<int>(value: 0, label: shortLabel),
+                ],
+                menuStyle: MenuStyle(),
+              ),
+            ),
+          ),
+        ),
+      );
+
+      await tester.tap(find.byType(TextField));
+      await tester.pumpAndSettle();
+
+      expect(tester.takeException(), isNull);
+      // Default width is 800, so the expected width is 800 - padding (20 + 20).
+      expect(tester.getSize(findMenuItemButton(shortLabel)).width, 760.0);
+    },
+  );
+
   // Regression test for https://github.com/flutter/flutter/issues/164905.
   testWidgets('ensure exclude semantics for trailing button', (WidgetTester tester) async {
     final SemanticsTester semantics = SemanticsTester(tester);