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.

Also added a test that checks if tapping empty space in channel header
area correctly navigates to the channel feed.

Fixes #1368.

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

lib/widgets/message_list.dart

@@ -1083,6 +1083,7 @@ 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 +1470,46 @@ void main() {
         ..status.equals(AnimationStatus.dismissed);
     });
   });
+
+  group('channel header navigation in multi-channel narrows', () {
+    for (final narrow in [
+      const CombinedFeedNarrow(),
+      const MentionsNarrow(),
+      const StarredMessagesNarrow(),
+    ]) {
+      testWidgets("navigates to ChannelNarrow when tapping empty space in channel header area in ${narrow.runtimeType}", (tester) async {
+        final pushedRoutes = <Route<void>>[];
+        final navObserver = TestNavigatorObserver()
+          ..onPushed = (route, prevRoute) => pushedRoutes.add(route);
+        final channel = eg.stream();
+        final message = eg.streamMessage(stream: channel);
+        await setupMessageListPage(tester,
+          narrow: narrow,
+          streams: [channel],
+          subscriptions: [eg.subscription(channel)],
+          messages: [message],
+          navObservers: [navObserver]);
+
+        assert(pushedRoutes.length == 1);
+        pushedRoutes.clear();
+
+        connection.prepare(json: eg.newestGetMessagesResult(
+          foundOldest: true, messages: [message]).toJson());
+
+        // Find the stream name text
+        final streamNameFinder = find.descendant(
+          of: find.byType(StreamMessageRecipientHeader),
+          matching: find.text(channel.name));
+
+        // Get the position of the stream name and tap 3px above it
+        final streamNameRect = tester.getRect(streamNameFinder);
+        await tester.tapAt(Offset(streamNameRect.center.dx, streamNameRect.top - 3));
+
+        await tester.pump();
+        check(pushedRoutes).single.isA<WidgetRoute>().page.isA<MessageListPage>()
+          .initNarrow.equals(ChannelNarrow(channel.streamId));
+        await tester.pumpAndSettle();
+      });
+    }
+  });
 }