msglist [nfc]: Introduce MarkAsReadAnimation widget

MarkAsReadAnimation is expected to get more complex in the upcoming
commits, so it's better to have it in a separate widget.

Author: Khader-1

lib/widgets/message_list.dart

@@ -507,10 +507,9 @@ class _MarkAsReadWidgetState extends State<MarkAsReadWidget> {
 
     return IgnorePointer(
       ignoring: areMessagesRead,
-      child: AnimatedOpacity(
-        opacity: areMessagesRead ? 0 : loading ? 0.5 : 1,
-        duration: Duration(milliseconds: areMessagesRead ? 2000 : 300),
-        curve: Curves.easeOut,
+      child: MarkAsReadAnimation(
+        loading: loading,
+        hidden: areMessagesRead,
         child: SizedBox(width: double.infinity,
           // Design referenced from:
           //   https://www.figma.com/file/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?type=design&node-id=132-9684&mode=design&t=jJwHzloKJ0TMOG4M-0
@@ -544,6 +543,28 @@ class _MarkAsReadWidgetState extends State<MarkAsReadWidget> {
   }
 }
 
+class MarkAsReadAnimation extends StatelessWidget {
+  final bool loading;
+  final bool hidden;
+  final Widget child;
+
+  const MarkAsReadAnimation({
+    super.key,
+    required this.loading,
+    required this.hidden,
+    required this.child
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    return AnimatedOpacity(
+      opacity: hidden ? 0 : loading ? 0.5 : 1,
+      duration: Duration(milliseconds: hidden ? 2000 : 300),
+      curve: Curves.easeOut,
+      child: child);
+  }
+}
+
 class RecipientHeader extends StatelessWidget {
   const RecipientHeader({super.key, required this.message, required this.narrow});
 

test/widgets/message_list_test.dart

@@ -901,6 +901,11 @@ void main() {
           check(opacity.opacity).equals(expected ? 0.5 : 1.0);
         }
 
+        void checkHasOpacityValue(WidgetTester tester, Widget child, double expectedOpacity) {
+          check(find.byWidget(child).evaluate()).length.equals(1);
+          check(tester.widget<AnimatedOpacity>(find.byType(AnimatedOpacity)).opacity).equals(expectedOpacity);
+        }
+
         testWidgets('loading is changed correctly', (WidgetTester tester) async {
           final narrow = TopicNarrow.ofMessage(message);
           await setupMessageListPage(tester,
@@ -945,6 +950,42 @@ void main() {
           await tester.pump(const Duration(milliseconds: 2000));
           checkAppearsLoading(tester, false);
         });
+
+        testWidgets('in idle state', (WidgetTester tester) async {
+          final child = Container();
+
+          await tester.pumpWidget(MaterialApp(
+            home: MarkAsReadAnimation(hidden: false, loading: false, child: child)));
+
+          checkHasOpacityValue(tester, child, 1);
+        });
+
+        testWidgets('in loading state', (WidgetTester tester) async {
+          final child = Container();
+
+          await tester.pumpWidget(MaterialApp(
+            home: MarkAsReadAnimation(hidden: false, loading: true, child: child)));
+
+          checkHasOpacityValue(tester, child, 0.5);
+        });
+
+        testWidgets('in hidden state', (WidgetTester tester) async {
+          final child = Container();
+
+          await tester.pumpWidget(MaterialApp(
+            home: MarkAsReadAnimation(hidden: true, loading: false, child: child)));
+
+          checkHasOpacityValue(tester, child, 0);
+        });
+
+        testWidgets('in hidden state but loading is true', (WidgetTester tester) async {
+          final child = Container();
+
+          await tester.pumpWidget(MaterialApp(
+            home: MarkAsReadAnimation(hidden: true, loading: true, child: child)));
+
+          checkHasOpacityValue(tester, child, 0);
+        });
       });
 
       testWidgets('smoke test on modern server', (WidgetTester tester) async {