msglist: Fix channel header tap behaviour in multi-channel narrows

Set gesture detecter behavior of streamWidget to HitTestBehavior.opaque
to handle taps in empty space around the header.

Fixes #1368.

Author: E-m-i-n-e-n-c-e

lib/widgets/message_list.dart

@@ -1082,6 +1082,7 @@ class StreamMessageRecipientHeader extends StatelessWidget {
         ?? zulipLocalizations.unknownChannelName; // TODO(log)
 
       streamWidget = GestureDetector(
+        behavior: HitTestBehavior.opaque,
         onTap: () => Navigator.push(context,
           MessageListPage.buildRoute(context: context,
             narrow: ChannelNarrow(message.streamId))),

test/widgets/message_list_test.dart

@@ -1470,4 +1470,112 @@ void main() {
         ..status.equals(AnimationStatus.dismissed);
     });
   });
+
+  group('recipient header navigation in multi-channel narrows', () {
+    late List<Route<void>> pushedRoutes;
+    late TestNavigatorObserver navObserver;
+    late ZulipStream channel;
+    late StreamMessage message;
+    late Finder recipientHeaderFinder;
+    late Rect recipientHeaderRect;
+
+    Future<void> prepare(WidgetTester tester, {String? topic}) async {
+      pushedRoutes = <Route<void>>[];
+      navObserver = TestNavigatorObserver()
+        ..onPushed = (route, prevRoute) => pushedRoutes.add(route);
+      channel = eg.stream();
+      message = topic == null
+        ? eg.streamMessage(stream: channel)
+        : eg.streamMessage(stream: channel, topic: topic);
+
+      await setupMessageListPage(tester,
+        narrow: const CombinedFeedNarrow(),
+        streams: [channel],
+        subscriptions: [eg.subscription(channel)],
+        messages: [message],
+        navObservers: [navObserver]);
+
+      assert(pushedRoutes.length == 1);
+      pushedRoutes.clear();
+
+      recipientHeaderFinder = find.byType(StreamMessageRecipientHeader);
+      recipientHeaderRect = tester.getRect(recipientHeaderFinder);
+    }
+
+    testWidgets("navigates to ChannelNarrow when tapping above or below channel name in recipient header",
+     (tester) async {
+      await prepare(tester);
+
+      final channelNameFinder = find.descendant(
+        of: recipientHeaderFinder,
+        matching: find.text(channel.name));
+      final channelNameRect = tester.getRect(channelNameFinder);
+
+      connection.prepare(json: eg.newestGetMessagesResult(
+      foundOldest: true, messages: [message]).toJson());
+      // First tap: 1 px below the top of recepient header
+      // at channelName x coordinate
+      await tester.tapAt(Offset(
+        channelNameRect.center.dx, recipientHeaderRect.top + 1));
+
+      await tester.pump();
+      check(pushedRoutes).single.isA<WidgetRoute>().page.isA<MessageListPage>()
+        .initNarrow.equals(ChannelNarrow(channel.streamId));
+      await tester.pumpAndSettle();
+
+      // Navigate back to original page and clear routes
+      await tester.pageBack();
+      await tester.pumpAndSettle();
+      pushedRoutes.clear();
+
+      connection.prepare(json: eg.newestGetMessagesResult(
+        foundOldest: true, messages: [message]).toJson());
+      // Second tap: 1 px above the bottom of the recipient header
+      // at channelName x coordinate
+      await tester.tapAt(Offset(
+        channelNameRect.center.dx, channelNameRect.bottom - 1));
+      await tester.pump();
+      check(pushedRoutes).single.isA<WidgetRoute>().page.isA<MessageListPage>()
+        .initNarrow.equals(ChannelNarrow(channel.streamId));
+      await tester.pumpAndSettle();
+    });
+
+    testWidgets("navigates to TopicNarrow when tapping above or below topic name in recipient header",
+     (tester) async {
+      const testTopic = 'testTopic';
+      await prepare(tester, topic: testTopic);
+
+      final topicNameFinder = find.descendant(
+        of: recipientHeaderFinder,
+        matching: find.text(testTopic));
+      final topicNameRect = tester.getRect(topicNameFinder);
+
+      connection.prepare(json: eg.newestGetMessagesResult(
+        foundOldest: true, messages: [message]).toJson());
+      // First tap: 1 px below the top of recepient header
+      // at topicName x coordinate
+      await tester.tapAt(Offset(
+        topicNameRect.center.dx, recipientHeaderRect.top + 1));
+      await tester.pump();
+      check(pushedRoutes).single.isA<WidgetRoute>().page.isA<MessageListPage>()
+        .initNarrow.equals(TopicNarrow(channel.streamId, message.topic));
+      await tester.pumpAndSettle();
+
+      // Navigate back to original page and clear routes
+      await tester.pageBack();
+      await tester.pumpAndSettle();
+      pushedRoutes.clear();
+
+      connection.prepare(json: eg.newestGetMessagesResult(
+        foundOldest: true, messages: [message]).toJson());
+      // Second tap: 1 px above the bottom of the recipient header
+      // at topicName x coordinate
+      await tester.tapAt(Offset(
+        topicNameRect.center.dx, recipientHeaderRect.bottom - 1));
+      await tester.pump();
+      check(pushedRoutes).single.isA<WidgetRoute>().page.isA<MessageListPage>()
+        .initNarrow.equals(TopicNarrow(channel.streamId, message.topic));
+      await tester.pumpAndSettle();
+    });
+  });
 }