combined feed: Fix channel header tap behaviour in combined feed

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 #1179.

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 +1470,38 @@ void main() {
         ..status.equals(AnimationStatus.dismissed);
     });
   });
+
+  testWidgets("navigates to ChannelNarrow when tapping the empty space in the channel part area of the CombinedFeedNarrow header", (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: const CombinedFeedNarrow(),
+      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 chevron icon
+    final chevronFinder = find.descendant(
+      of: find.byType(StreamMessageRecipientHeader),
+      matching: find.byIcon(ZulipIcons.chevron_right));
+
+    // Get the position of the chevron icon and tap 3px above it
+    final chevronRect = tester.getRect(chevronFinder);
+    await tester.tapAt(Offset(chevronRect.center.dx, chevronRect.top - 3));
+
+    await tester.pump();
+    check(pushedRoutes).single.isA<WidgetRoute>().page.isA<MessageListPage>()
+      .initNarrow.equals(ChannelNarrow(channel.streamId));
+    await tester.pumpAndSettle();
+  });
 }