narrow [nfc]: Make Narrow.containsMessage return nullable bool

Done as preparation to implement a new Narrow subclass for keyword
search.

We can't really implement a client-side "contains-message" predicate
for search; it's up to the server to decide whether a message
matches the search query.

A simple path to representing that, done here, is to just change
Narrow.containsMessage's contract to say that it always returns null
if the narrow is a search narrow.

The app-code changes show just small behavior choices for a
message-list page in a search narrow, which we'll implement soon:
- Message events are ignored
- Outbox messages never appear

Various TODOs in tests; we'll handle all of those as part of adding
the keyword-search Narrow subclass.

Author: chrisbobbe

lib/model/message_list.dart

@@ -939,7 +939,7 @@ class MessageListView with ChangeNotifier, _MessageSequence {
   bool _shouldAddOutboxMessage(OutboxMessage outboxMessage) {
     assert(haveNewest);
     return !outboxMessage.hidden
-      && narrow.containsMessage(outboxMessage)
+      && narrow.containsMessage(outboxMessage) == true
       && _messageVisible(outboxMessage);
   }
 
@@ -1026,7 +1026,7 @@ class MessageListView with ChangeNotifier, _MessageSequence {
   /// Add [MessageEvent.message] to this view, if it belongs here.
   void handleMessageEvent(MessageEvent event) {
     final message = event.message;
-    if (!narrow.containsMessage(message) || !_messageVisible(message)) {
+    if (narrow.containsMessage(message) != true || !_messageVisible(message)) {
       assert(event.localMessageId == null || outboxMessages.none((message) =>
         message.localMessageId == int.parse(event.localMessageId!, radix: 10)));
       return;

lib/model/narrow.dart

@@ -19,7 +19,10 @@ sealed class Narrow {
   /// This does not necessarily mean the message list would show this message
   /// when navigated to this narrow; in particular it does not address the
   /// question of whether the stream or topic, or the sending user, is muted.
-  bool containsMessage(MessageBase message);
+  ///
+  /// Null when the client is unable to predict whether the message
+  /// satisfies the filters of this narrow, e.g. when this is a search narrow.
+  bool? containsMessage(MessageBase message);
 
   /// This narrow, expressed as an [ApiNarrow].
   ApiNarrow apiEncode();

lib/widgets/message_list.dart

@@ -587,7 +587,7 @@ class MessageListAppBarTitle extends StatelessWidget {
                 // either still fetching messages (and the user can reopen the
                 // sheet after that finishes) or there aren't any messages to
                 // act on anyway.
-                assert(someMessage == null || narrow.containsMessage(someMessage));
+                assert(someMessage == null || narrow.containsMessage(someMessage)!);
                 showTopicActionSheet(context,
                   channelId: streamId,
                   topic: topic,

test/model/message_list_test.dart

@@ -3001,7 +3001,9 @@ void checkInvariants(MessageListView model) {
   final allMessages = [...model.messages, ...model.outboxMessages];
 
   for (final message in allMessages) {
-    check(model.narrow.containsMessage(message)).isTrue();
+    check(model.narrow.containsMessage(message))
+      .isNotNull() // TODO allow/expect null if narrow is a search narrow.
+      .isTrue();
 
     if (message is! MessageBase<StreamConversation>) continue;
     final conversation = message.conversation;

test/widgets/action_sheet_test.dart

@@ -63,7 +63,8 @@ Future<void> setupToMessageActionSheet(WidgetTester tester, {
   Future<void> Function()? beforeLongPress,
 }) async {
   addTearDown(testBinding.reset);
-  assert(narrow.containsMessage(message));
+  // TODO(#1667) will be null in a search narrow; remove `!`.
+  assert(narrow.containsMessage(message)!);
 
   await testBinding.globalStore.add(
     eg.selfAccount,

test/widgets/compose_box_test.dart

@@ -1702,7 +1702,10 @@ void main() {
 
     Message msgInNarrow(Narrow narrow) {
       final List<Message> messages = [message, dmMessage];
-      return messages.where((m) => narrow.containsMessage(m)).single;
+      return messages.where(
+        // TODO(#1667) will be null in a search narrow; remove `!`.
+        (m) => narrow.containsMessage(m)!
+      ).single;
     }
 
     int msgIdInNarrow(Narrow narrow) => msgInNarrow(narrow).id;

test/widgets/emoji_reaction_test.dart

@@ -330,7 +330,8 @@ void main() {
       required Narrow narrow,
     }) async {
       addTearDown(testBinding.reset);
-      assert(narrow.containsMessage(message));
+      // TODO(#1667) will be null in a search narrow; remove `!`.
+      assert(narrow.containsMessage(message)!);
 
       final httpClient = FakeImageHttpClient();
       debugNetworkImageHttpClientProvider = () => httpClient;