fix: selected date decorator renders outside PageView in DatePickerDialog dialog (flutter#171718)

Wraps `PageView` with a transparent `Material` to ensure the selected
date decorator is painted in the correct context, avoiding visual
glitches during transitions.

Fixes flutter#171717 

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [ ] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

---------

Co-authored-by: Tong Mu <[email protected]>
Co-authored-by: Huy <[email protected]>

Author: 3 people

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

@@ -21,6 +21,7 @@ import 'icon_button.dart';
 import 'icons.dart';
 import 'ink_decoration.dart';
 import 'ink_well.dart';
+import 'material.dart';
 import 'material_localizations.dart';
 import 'material_state.dart';
 import 'theme.dart';
@@ -888,13 +889,18 @@ class _MonthPickerState extends State<_MonthPicker> {
               child: _FocusedDate(
                 calendarDelegate: widget.calendarDelegate,
                 date: _dayGridFocus.hasFocus ? _focusedDay : null,
-                child: PageView.builder(
-                  key: _pageViewKey,
-                  controller: _pageController,
-                  itemBuilder: _buildItems,
-                  itemCount:
-                      widget.calendarDelegate.monthDelta(widget.firstDate, widget.lastDate) + 1,
-                  onPageChanged: _handleMonthPageChanged,
+                // Wrap the PageView with `Material`, so when its child paints on materials
+                // the content won't go out of boundary during page transition.
+                child: Material(
+                  type: MaterialType.transparency,
+                  child: PageView.builder(
+                    key: _pageViewKey,
+                    controller: _pageController,
+                    itemBuilder: _buildItems,
+                    itemCount:
+                        widget.calendarDelegate.monthDelta(widget.firstDate, widget.lastDate) + 1,
+                    onPageChanged: _handleMonthPageChanged,
+                  ),
                 ),
               ),
             ),

packages/flutter/test/material/calendar_date_picker_test.dart

@@ -1435,6 +1435,29 @@ void main() {
         ),
       );
     });
+
+    testWidgets('Ink feature paints on inner Material', (WidgetTester tester) async {
+      await tester.pumpWidget(
+        calendarDatePicker(
+          firstDate: DateTime(2025, DateTime.june),
+          initialDate: DateTime(2025, DateTime.july, 20),
+          lastDate: DateTime(2025, DateTime.august, 31),
+        ),
+      );
+
+      // Material outside the PageView.
+      final MaterialInkController outerMaterial = Material.of(
+        tester.element(find.byType(FocusableActionDetector)),
+      );
+      // Material directly wrapping the PageView.
+      final MaterialInkController innerMaterial = Material.of(
+        tester.element(find.byType(PageView)),
+      );
+
+      // Only the inner Material should have ink features.
+      expect((outerMaterial as dynamic).debugInkFeatures, isNull);
+      expect((innerMaterial as dynamic).debugInkFeatures, hasLength(31));
+    });
   });
 
   group('YearPicker', () {