msglist: On tap recipient header, open msglist anchored at message

Fixes: zulip#1621
Related: zulip#252

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -1437,7 +1437,8 @@ class StreamMessageRecipientHeader extends StatelessWidget {
       streamWidget = GestureDetector(
         onTap: () => Navigator.push(context,
           MessageListPage.buildRoute(context: context,
-            narrow: ChannelNarrow(streamId))),
+            narrow: ChannelNarrow(streamId),
+            initAnchorMessageId: message.id)),
         onLongPress: () => showChannelActionSheet(context, channelId: streamId),
         child: Row(
           crossAxisAlignment: CrossAxisAlignment.center,
@@ -1495,7 +1496,8 @@ class StreamMessageRecipientHeader extends StatelessWidget {
       onTap: narrow is TopicNarrow ? null
         : () => Navigator.push(context,
             MessageListPage.buildRoute(context: context,
-              narrow: TopicNarrow.ofMessage(message))),
+              narrow: TopicNarrow.ofMessage(message),
+              initAnchorMessageId: message.id)),
       onLongPress: () => showTopicActionSheet(context,
         channelId: streamId,
         topic: topic,
@@ -1552,7 +1554,8 @@ class DmRecipientHeader extends StatelessWidget {
       onTap: narrow is DmNarrow ? null
         : () => Navigator.push(context,
             MessageListPage.buildRoute(context: context,
-              narrow: DmNarrow.ofMessage(message, selfUserId: store.selfUserId))),
+              narrow: DmNarrow.ofMessage(message, selfUserId: store.selfUserId),
+              initAnchorMessageId: message.id)),
       child: ColoredBox(
         color: messageListTheme.dmRecipientHeaderBg,
         child: Padding(

test/widgets/message_list_checks.dart

@@ -4,4 +4,5 @@ import 'package:zulip/widgets/message_list.dart';
 
 extension MessageListPageChecks on Subject<MessageListPage> {
   Subject<Narrow> get initNarrow => has((x) => x.initNarrow, 'initNarrow');
+  Subject<int?> get initAnchorMessageId => has((x) => x.initAnchorMessageId, 'initAnchorMessageId');
 }

test/widgets/message_list_test.dart

@@ -1365,7 +1365,8 @@ void main() {
           matching: find.text(channel.name)));
         await tester.pump();
         check(pushedRoutes).single.isA<WidgetRoute>().page.isA<MessageListPage>()
-          .initNarrow.equals(ChannelNarrow(channel.streamId));
+          ..initNarrow.equals(ChannelNarrow(channel.streamId))
+          ..initAnchorMessageId.isNotNull().equals(message.id);
         await tester.pumpAndSettle();
       });
 
@@ -1391,7 +1392,8 @@ void main() {
           matching: find.text('topic name')));
         await tester.pump();
         check(pushedRoutes).single.isA<WidgetRoute>().page.isA<MessageListPage>()
-          .initNarrow.equals(TopicNarrow.ofMessage(message));
+          ..initNarrow.equals(TopicNarrow.ofMessage(message))
+          ..initAnchorMessageId.isNotNull().equals(message.id);
         await tester.pumpAndSettle();
       });
 
@@ -1500,7 +1502,8 @@ void main() {
       await tester.tap(find.byType(DmRecipientHeader));
       await tester.pump();
       check(pushedRoutes).single.isA<WidgetRoute>().page.isA<MessageListPage>()
-        .initNarrow.equals(DmNarrow.withUser(eg.otherUser.userId, selfUserId: eg.selfUser.userId));
+        ..initNarrow.equals(DmNarrow.withUser(eg.otherUser.userId, selfUserId: eg.selfUser.userId))
+        ..initAnchorMessageId.isNotNull().equals(dmMessage.id);
       await tester.pumpAndSettle();
     });