Fix Cupertino Context Menu Container to Remove White Corners (#144883)

Fixes a visual glitch when CupertinoContextMenu is on a non-white background.

packages/flutter/lib/src/cupertino/context_menu.dart

@@ -172,11 +172,9 @@ class CupertinoContextMenu extends StatefulWidget {
   ///
   ///     final Animation<Decoration> boxDecorationAnimation = DecorationTween(
   ///       begin: const BoxDecoration(
-  ///        color: Color(0xFFFFFFFF),
   ///        boxShadow: <BoxShadow>[],
   ///       ),
   ///       end: const BoxDecoration(
-  ///        color: Color(0xFFFFFFFF),
   ///        boxShadow: CupertinoContextMenu.kEndBoxShadow,
   ///       ),
   ///      ).animate(
@@ -279,11 +277,9 @@ class CupertinoContextMenu extends StatefulWidget {
   ///
   ///     final Animation<Decoration> boxDecorationAnimation = DecorationTween(
   ///       begin: const BoxDecoration(
-  ///        color: Color(0xFFFFFFFF),
   ///        boxShadow: <BoxShadow>[],
   ///       ),
   ///       end: const BoxDecoration(
-  ///        color: Color(0xFFFFFFFF),
   ///        boxShadow: CupertinoContextMenu.kEndBoxShadow,
   ///       ),
   ///      ).animate(
@@ -676,11 +672,9 @@ class _DecoyChildState extends State<_DecoyChild> with TickerProviderStateMixin
 
     _boxDecoration = DecorationTween(
       begin: const BoxDecoration(
-        color: Color(0xFFFFFFFF),
         boxShadow: <BoxShadow>[],
       ),
       end: const BoxDecoration(
-        color: Color(0xFFFFFFFF),
         boxShadow: _endBoxShadow,
       ),
     ).animate(CurvedAnimation(

packages/flutter/test/cupertino/context_menu_test.dart

@@ -241,6 +241,61 @@ void main() {
       expect(findStatic(), findsOneWidget);
     });
 
+    testWidgets('_DecoyChild preserves the child color', (WidgetTester tester) async {
+      final Widget child = getChild();
+      await tester.pumpWidget(CupertinoApp(
+        home: CupertinoPageScaffold(
+          backgroundColor: CupertinoColors.black,
+            child: MediaQuery(
+              data: const MediaQueryData(size: Size(800, 600)),
+                child: Center(
+                  child: CupertinoContextMenu(
+                  actions: const <CupertinoContextMenuAction>[
+                    CupertinoContextMenuAction(
+                      child: Text('CupertinoContextMenuAction'),
+                    ),
+                  ],
+                child: child
+              ),
+            )
+          )
+        ),
+      ));
+
+      // Expect no _DecoyChild to be present before the gesture.
+      final Finder decoyChild = find
+          .byWidgetPredicate((Widget w) => '${w.runtimeType}' == '_DecoyChild');
+      expect(decoyChild, findsNothing);
+
+      // Start press gesture on the child.
+      final Rect childRect = tester.getRect(find.byWidget(child));
+      final TestGesture gesture = await tester.startGesture(childRect.center);
+      await tester.pump();
+
+      // Find the _DecoyChild by runtimeType,
+      // find the Container descendant with the BoxDecoration,
+      // then read the boxDecoration property.
+      final Finder decoyChildDescendant = find.descendant(
+          of: decoyChild,
+          matching: find.byType(Container));
+      final BoxDecoration? boxDecoration = (tester.firstWidget(decoyChildDescendant) as Container).decoration as BoxDecoration?;
+      const List<Color?> expectedColors = <Color?>[null, Color(0x00000000)];
+
+      // `Color(0x00000000)` -> Is `Colors.transparent`.
+      // `null`              -> Default when no color argument is given in `BoxDecoration`.
+      // Any other color won't preserve the child's property.
+      expect(expectedColors, contains(boxDecoration?.color));
+
+      // End the gesture.
+      await gesture.up();
+      await tester.pumpAndSettle();
+
+      // Expect no _DecoyChild to be present after ending the gesture.
+      final Finder decoyChildAfterEnding = find
+          .byWidgetPredicate((Widget w) => '${w.runtimeType}' == '_DecoyChild');
+      expect(decoyChildAfterEnding, findsNothing);
+    });
+
     testWidgets('CupertinoContextMenu with a basic builder opens and closes the same as when providing a child', (WidgetTester tester) async {
       final Widget child = getChild();
       await tester.pumpWidget(getBuilderContextMenu(builder: (BuildContext context, Animation<double> animation) {