Handle moved messages for unreads

Fixes #901. (**edit**, after splitting that issue.)

Work toward #901

I prioritized updating unreads data for moved messages because it is more visible after supporting the resolve/unresolve topic actions. Updating recent senders data will be a follow-up to this.

Handle moved messages in unreads data

Handle moved messages in unreads data
With #787 we implemented most of #150, handling moved messages. But as that PR description said:
> Fixes most of https://github.com/zulip/zulip-flutter/issues/150. (After this lands we should do a quick scan to identify anything else that needs updating on message moves — there are several new data structures in the store that we've merged in recent weeks.)

Then we didn't do that scan.

Places that still need updating on message moves include:
- [x] `Unreads` (which has a TODO comment for it)
- [x] ~~`RecentSenders` (that's one of those recently-merged data structures)~~ → split to its own issue: #1460
- … OK and I think that's it, having now scanned through what's in the store. (Another recently-merged data structure is `RecentDmConversationsView`, but that's only about DMs and those can't be moved.)

Related issues:

- #862

The key user-facing scenario where this comes up, which I happened to see again earlier today:
* You're reading messages in Combined feed.
* A topic gets resolved, containing some unread messages.
* The messages are still unread, and shown with the blue unread marker at left. If you tap the recipient header to navigate to their conversation, the unread marker appears, as it should.
* But the conversation *doesn't* have a mark-as-read button. As a result there's no way for you to dismiss those unread markers (short of marking read the entire combined feed, or entire channel).

What's happening is that the `Unreads` data structure still has the messages under their old, unresolved topic; so when viewing the new resolved topic, it finds no unreads for that topic.

I will work on this next as with #1301 this scenario might come up more often.
`TypingStatus` also refers to data that can be moved (`TopicNarrow`s), but it appears that the web app doesn't move an active typing indicator even if propagate mode is 'all'. It could be a deliberate UX or a known/unknown issue. Anyway, this is pretty subtle and we shouldn't need to consider it when working on this.

flutter test test/api/model/events_test.dart test/model/message_list_test.dart test/model/unreads_test.dart --reporter expanded 2>&1

diff --git a/lib/api/model/events.dart b/lib/api/model/events.dart
index 9faa3d367e..985f5b5803 100644
--- a/lib/api/model/events.dart
+++ b/lib/api/model/events.dart
@@ -718,16 +718,8 @@ class UpdateMessageEvent extends Event {

   // final String? streamName; // ignore

-  @JsonKey(name: 'stream_id')
-  final int? origStreamId;
-  final int? newStreamId;
-
-  final PropagateMode? propagateMode;
-
-  @JsonKey(name: 'orig_subject')
-  final TopicName? origTopic;
-  @JsonKey(name: 'subject')
-  final TopicName? newTopic;
+  @JsonKey(readValue: _readMoveData, fromJson: UpdateMessageMoveData.tryParseFromJson, includeToJson: false)
+  final UpdateMessageMoveData? moveData;

   // final List<TopicLink> topicLinks; // TODO handle

@@ -747,11 +739,7 @@ class UpdateMessageEvent extends Event {
     required this.messageIds,
     required this.flags,
     required this.editTimestamp,
-    required this.origStreamId,
-    required this.newStreamId,
-    required this.propagateMode,
-    required this.origTopic,
-    required this.newTopic,
+    required this.moveData,
     required this.origContent,
     required this.origRenderedContent,
     required this.content,
@@ -759,6 +747,12 @@ class UpdateMessageEvent extends Event {
     required this.isMeMessage,
   });

+  static Map<String, dynamic> _readMoveData(Map<dynamic, dynamic> json, String key) {
+    // Parsing [UpdateMessageMoveData] requires `json`, not the default `json[key]`.
+    assert(json is Map<String, dynamic>); // value came through `fromJson` with this type
+    return json as Map<String, dynamic>;
+  }
+
   factory UpdateMessageEvent.fromJson(Map<String, dynamic> json) =>
     _$UpdateMessageEventFromJson(json);

@@ -766,6 +760,72 @@ class UpdateMessageEvent extends Event {
   Map<String, dynamic> toJson() => _$UpdateMessageEventToJson(this);
 }

+/// Data structure representing a message move.
+class UpdateMessageMoveData {
+  final int origStreamId;
+  final int newStreamId;
+  final TopicName origTopic;
+  final TopicName newTopic;
+  final PropagateMode propagateMode;
+
+  UpdateMessageMoveData({
+    required this.origStreamId,
+    required this.newStreamId,
+    required this.origTopic,
+    required this.newTopic,
+    required this.propagateMode,
+  }) : assert(newStreamId != origStreamId || newTopic != origTopic);
+
+  /// Try to extract [UpdateMessageMoveData] from the JSON object for an
+  /// [UpdateMessageEvent].
+  ///
+  /// Returns `null` if there was no message move.
+  ///
+  /// Throws an error if the data is malformed.
+  // When parsing this, 'stream_id', which is also present when there was only
+  // a content edit, cannot be recovered if this ends up returning `null`.
+  // This may matter if we ever need 'stream_id' when no message move occurred.
+  static UpdateMessageMoveData? tryParseFromJson(Map<String, Object?> json) {
+    final origStreamId = (json['stream_id'] as num?)?.toInt();
+    final newStreamIdRaw = (json['new_stream_id'] as num?)?.toInt();
+    final newStreamId = newStreamIdRaw ?? origStreamId;
+
+    final origTopic = json['orig_subject'] == null ? null
+      : TopicName.fromJson(json['orig_subject'] as String);
+    final newTopicRaw = json['subject'] == null ? null
+      : TopicName.fromJson(json['subject'] as String);
+    final newTopic = newTopicRaw ?? origTopic;
+
+    final propagateModeString = json['propagate_mode'] as String?;
+    final propagateMode = propagateModeString == null ? null
+      : PropagateMode.fromRawString(propagateModeString);
+
+    if (newStreamId == origStreamId && newTopic == origTopic) {
+      if (propagateMode != null) {
+        throw FormatException(
+          'Malformed UpdateMessageEvent: incoherent message-move fields; '
+          'propagate_mode present but no new channel or topic');
+      }
+      return null;
+    }
+
+    return UpdateMessageMoveData(
+      // The `stream_id` field (aka origStreamId) is documented to be present on moves;
+      // newStreamId should not be null either because it falls back to origStreamId.
+      origStreamId: origStreamId!,
+      newStreamId: newStreamId!,
+
+      // The `orig_subject` field (aka origTopic) is documented to be present on moves;
+      // newTopic should not be null either because it falls back to origTopic.
+      origTopic: origTopic!,
+      newTopic: newTopic!,
+
+      // The `propagate_mode` field (aka propagateMode) is documented to be present on moves.
+      propagateMode: propagateMode!,
+    );
+  }
+}
+
 /// A Zulip event of type `delete_message`: https://zulip.com/api/get-events#delete_message
 @JsonSerializable(fieldRename: FieldRename.snake)
 class DeleteMessageEvent extends Event {
diff --git a/lib/api/model/events.g.dart b/lib/api/model/events.g.dart
index d0b0cc7b1b..c5b56d013a 100644
--- a/lib/api/model/events.g.dart
+++ b/lib/api/model/events.g.dart
@@ -443,20 +443,10 @@ UpdateMessageEvent _$UpdateMessageEventFromJson(Map<String, dynamic> json) =>
               .map((e) => $enumDecode(_$MessageFlagEnumMap, e))
               .toList(),
       editTimestamp: (json['edit_timestamp'] as num?)?.toInt(),
-      origStreamId: (json['stream_id'] as num?)?.toInt(),
-      newStreamId: (json['new_stream_id'] as num?)?.toInt(),
-      propagateMode: $enumDecodeNullable(
-        _$PropagateModeEnumMap,
-        json['propagate_mode'],
+      moveData: UpdateMessageMoveData.tryParseFromJson(
+        UpdateMessageEvent._readMoveData(json, 'move_data')
+            as Map<String, Object?>,
       ),
-      origTopic:
-          json['orig_subject'] == null
-              ? null
-              : TopicName.fromJson(json['orig_subject'] as String),
-      newTopic:
-          json['subject'] == null
-              ? null
-              : TopicName.fromJson(json['subject'] as String),
       origContent: json['orig_content'] as String?,
       origRenderedContent: json['orig_rendered_content'] as String?,
       content: json['content'] as String?,
@@ -474,11 +464,6 @@ Map<String, dynamic> _$UpdateMessageEventToJson(UpdateMessageEvent instance) =>
       'message_ids': instance.messageIds,
       'flags': instance.flags,
       'edit_timestamp': instance.editTimestamp,
-      'stream_id': instance.origStreamId,
-      'new_stream_id': instance.newStreamId,
-      'propagate_mode': instance.propagateMode,
-      'orig_subject': instance.origTopic,
-      'subject': instance.newTopic,
       'orig_content': instance.origContent,
       'orig_rendered_content': instance.origRenderedContent,
       'content': instance.content,
@@ -497,12 +482,6 @@ const _$MessageFlagEnumMap = {
   MessageFlag.unknown: 'unknown',
 };

-const _$PropagateModeEnumMap = {
-  PropagateMode.changeOne: 'change_one',
-  PropagateMode.changeLater: 'change_later',
-  PropagateMode.changeAll: 'change_all',
-};
-
 DeleteMessageEvent _$DeleteMessageEventFromJson(Map<String, dynamic> json) =>
     DeleteMessageEvent(
       id: (json['id'] as num).toInt(),
diff --git a/lib/api/model/model.dart b/lib/api/model/model.dart
index fad8ddc5bc..5c94626584 100644
--- a/lib/api/model/model.dart
+++ b/lib/api/model/model.dart
@@ -954,4 +954,15 @@ enum PropagateMode {
   changeAll;

   String toJson() => _$PropagateModeEnumMap[this]!;
+
+  /// Get a [PropagateMode] from a raw string. Throws if the string is
+  /// unrecognized.
+  ///
+  /// Example:
+  ///   'change_one' -> PropagateMode.changeOne
+  static PropagateMode fromRawString(String raw) => _byRawString[raw]!;
+
+  // _$…EnumMap is thanks to `alwaysCreate: true` and `fieldRename: FieldRename.snake`
+  static final _byRawString = _$PropagateModeEnumMap
+    .map((key, value) => MapEntry(value, key));
 }
diff --git a/lib/model/message.dart b/lib/model/message.dart
index 84f3bbc3e5..d1886b810f 100644
--- a/lib/model/message.dart
+++ b/lib/model/message.dart
@@ -169,48 +169,17 @@ class MessageStoreImpl with MessageStore {
   }

   void _handleUpdateMessageEventMove(UpdateMessageEvent event) {
-    // The interaction between the fields of these events are a bit tricky.
-    // For reference, see: https://zulip.com/api/get-events#update_message
-
-    final origStreamId = event.origStreamId;
-    final newStreamId = event.newStreamId; // null if topic-only move
-    final origTopic = event.origTopic;
-    final newTopic = event.newTopic;
-    final propagateMode = event.propagateMode;
-
-    if (origTopic == null) {
+    final messageMove = event.moveData;
+    if (messageMove == null) {
       // There was no move.
-      assert(() {
-        if (newStreamId != null && origStreamId != null
-            && newStreamId != origStreamId) {
-          // This should be impossible; `orig_subject` (aka origTopic) is
-          // documented to be present when either the stream or topic changed.
-          debugLog('Malformed UpdateMessageEvent: stream move but no origTopic'); // TODO(log)
-        }
-        return true;
-      }());
       return;
     }

-    if (newStreamId == null && newTopic == null) {
-      // If neither the channel nor topic name changed, nothing moved.
-      // In that case `orig_subject` (aka origTopic) should have been null.
-      assert(debugLog('Malformed UpdateMessageEvent: move but no newStreamId or newTopic')); // TODO(log)
-      return;
-    }
-    if (origStreamId == null) {
-      // The `stream_id` field (aka origStreamId) is documented to be present on moves.
-      assert(debugLog('Malformed UpdateMessageEvent: move but no origStreamId')); // TODO(log)
-      return;
-    }
-    if (propagateMode == null) {
-      // The `propagate_mode` field (aka propagateMode) is documented to be present on moves.
-      assert(debugLog('Malformed UpdateMessageEvent: move but no propagateMode')); // TODO(log)
-      return;
-    }
+    final UpdateMessageMoveData(
+      :origStreamId, :newStreamId, :origTopic, :newTopic) = messageMove;

-    final wasResolveOrUnresolve = (newStreamId == null
-      && MessageEditState.topicMoveWasResolveOrUnresolve(origTopic, newTopic!));
+    final wasResolveOrUnresolve = newStreamId == origStreamId
+      && MessageEditState.topicMoveWasResolveOrUnresolve(origTopic, newTopic);

     for (final messageId in event.messageIds) {
       final message = messages[messageId];
@@ -221,14 +190,14 @@ class MessageStoreImpl with MessageStore {
         continue;
       }

-      if (newStreamId != null) {
+      if (newStreamId != origStreamId) {
         message.streamId = newStreamId;
         // See [StreamMessage.displayRecipient] on why the invalidation is
         // needed.
         message.displayRecipient = null;
       }

-      if (newTopic != null) {
+      if (newTopic != origTopic) {
         message.topic = newTopic;
       }

@@ -239,14 +208,7 @@ class MessageStoreImpl with MessageStore {
     }

     for (final view in _messageListViews) {
-      view.messagesMoved(
-        origStreamId: origStreamId,
-        newStreamId: newStreamId ?? origStreamId,
-        origTopic: origTopic,
-        newTopic: newTopic ?? origTopic,
-        messageIds: event.messageIds,
-        propagateMode: propagateMode,
-      );
+      view.messagesMoved(messageMove: messageMove, messageIds: event.messageIds);
     }
   }

diff --git a/lib/model/message_list.dart b/lib/model/message_list.dart
index ea120cd72e..a1df255cb2 100644
--- a/lib/model/message_list.dart
+++ b/lib/model/message_list.dart
@@ -720,13 +720,12 @@ class MessageListView with ChangeNotifier, _MessageSequence {
   }

   void messagesMoved({
-    required int origStreamId,
-    required int newStreamId,
-    required TopicName origTopic,
-    required TopicName newTopic,
+    required UpdateMessageMoveData messageMove,
     required List<int> messageIds,
-    required PropagateMode propagateMode,
   }) {
+    final UpdateMessageMoveData(
+      :origStreamId, :newStreamId, :origTopic, :newTopic, :propagateMode,
+    ) = messageMove;
     switch (narrow) {
       case DmNarrow():
         // DMs can't be moved (nor created by moves),
diff --git a/lib/model/unreads.dart b/lib/model/unreads.dart
index 1fcea3f83c..34b4492013 100644
--- a/lib/model/unreads.dart
+++ b/lib/model/unreads.dart
@@ -259,10 +259,8 @@ class Unreads extends ChangeNotifier {
       (f) => f == MessageFlag.mentioned || f == MessageFlag.wildcardMentioned,
     );

-    // We assume this event can't signal a change in a message's 'read' flag.
-    // TODO can it actually though, when it's about messages being moved into an
-    //   unsubscribed stream?
-    //   https://chat.zulip.org/#narrow/stream/378-api-design/topic/mark-as-read.20events.20with.20message.20moves.3F/near/1639957
+    // We expect the event's 'read' flag to be boring,
+    // matching the message's local unread state.
     final bool isRead = event.flags.contains(MessageFlag.read);
     assert(() {
       final isUnreadLocally = isUnread(messageId);
@@ -272,6 +270,17 @@ class Unreads extends ChangeNotifier {
       // We were going to check something but can't; shrug.
       if (isUnreadLocally == null) return true;

+      final newChannelId = event.moveData?.newStreamId;
+      if (newChannelId != null && !channelStore.subscriptions.containsKey(newChannelId)) {
+        // When unread messages are moved to an unsubscribed channel, the server
+        // marks them as read without sending a mark-as-read event. Clients are
+        // asked to special-case this by marking them as read, which we do in
+        // _handleMessageMove. That contract is clear enough and doesn't involve
+        // this event's 'read' flag, so don't bother logging about the flag;
+        // its behavior seems like an implementation detail that could change.
+        return true;
+      }
+
       if (isUnreadLocally != isUnreadInEvent) {
         // If this happens, then either:
         // - the server and client have been out of sync about the message's
@@ -296,13 +305,39 @@ class Unreads extends ChangeNotifier {
         madeAnyUpdate |= mentions.add(messageId);
     }

-    // TODO(#901) handle moved messages
+    madeAnyUpdate |= _handleMessageMove(event);

     if (madeAnyUpdate) {
       notifyListeners();
     }
   }

+  bool _handleMessageMove(UpdateMessageEvent event) {
+    if (event.moveData == null) {
+      // No moved messages.
+      return false;
+    }
+    final UpdateMessageMoveData(
+      :origStreamId, :newStreamId, :origTopic, :newTopic) = event.moveData!;
+
+    final messageToMoveIds = _popAllInStreamTopic(
+      event.messageIds.toSet(), origStreamId, origTopic)?..sort();
+
+    if (messageToMoveIds == null || messageToMoveIds.isEmpty) return false;
+    assert(event.messageIds.toSet().containsAll(messageToMoveIds));
+
+    if (!channelStore.subscriptions.containsKey(newStreamId)) {
+      // Unreads moved to an unsubscribed channel; just drop them.
+      // See also:
+      //   https://chat.zulip.org/#narrow/channel/378-api-design/topic/mark-as-read.20events.20with.20message.20moves.3F/near/2101926
+      return true;
+    }
+
+    _addAllInStreamTopic(messageToMoveIds, newStreamId, newTopic);
+
+    return true;
+  }
+
   void handleDeleteMessageEvent(DeleteMessageEvent event) {
     mentions.removeAll(event.messageIds);
     final messageIdsSet = Set.of(event.messageIds);
@@ -455,6 +490,8 @@ class Unreads extends ChangeNotifier {

   // [messageIds] must be sorted ascending and without duplicates.
   void _addAllInStreamTopic(QueueList<int> messageIds, int streamId, TopicName topic) {
+    assert(messageIds.isNotEmpty);
+    assert(isSortedWithoutDuplicates(messageIds));
     final topics = streams[streamId] ??= {};
     topics.update(topic,
       ifAbsent: () => messageIds,
@@ -504,6 +541,49 @@ class Unreads extends ChangeNotifier {
     }
   }

+  /// Remove unread stream messages contained in `incomingMessageIds`, with
+  /// the matching `streamId` and `topic`.
+  ///
+  /// Returns the removed message IDs, or `null` if no messages are affected.
+  ///
+  /// Use [_removeAllInStreamTopic] if the removed message IDs are not needed.
+  // Part of this is adapted from [ListBase.removeWhere].
+  QueueList<int>? _popAllInStreamTopic(Set<int> incomingMessageIds, int streamId, TopicName topic) {
+    final topics = streams[streamId];
+    if (topics == null) return null;
+    final messageIds = topics[topic];
+    if (messageIds == null) return null;
+
+    final retainedMessageIds = messageIds.whereNot(
+      (id) => incomingMessageIds.contains(id)).toList();
+
+    if (retainedMessageIds.isEmpty) {
+      // This is an optimization for the case when all messages in the
+      // conversation are removed, which avoids making a copy of `messageIds`
+      // unnecessarily.
+      topics.remove(topic);
+      if (topics.isEmpty) {
+        streams.remove(streamId);
+      }
+      return messageIds;
+    }
+
+    QueueList<int>? poppedMessageIds;
+    if (retainedMessageIds.length != messageIds.length) {
+      poppedMessageIds = QueueList.from(
+        messageIds.where((id) => incomingMessageIds.contains(id)));
+      messageIds.setRange(0, retainedMessageIds.length, retainedMessageIds);
+      messageIds.length = retainedMessageIds.length;
+    }
+    if (messageIds.isEmpty) {
+      topics.remove(topic);
+      if (topics.isEmpty) {
+        streams.remove(streamId);
+      }
+    }
+    return poppedMessageIds;
+  }
+
   // TODO use efficient model lookups
   bool _slowIsPresentInDms(int messageId) {
     return dms.values.any((ids) => ids.contains(messageId));

diff --git a/test/api/model/events_checks.dart b/test/api/model/events_checks.dart
index c1fa0a117f..12d74479de 100644
--- a/test/api/model/events_checks.dart
+++ b/test/api/model/events_checks.dart
@@ -48,11 +48,7 @@ extension UpdateMessageEventChecks on Subject<UpdateMessageEvent> {
   Subject<List<int>> get messageIds => has((e) => e.messageIds, 'messageIds');
   Subject<List<MessageFlag>> get flags => has((e) => e.flags, 'flags');
   Subject<int?> get editTimestamp => has((e) => e.editTimestamp, 'editTimestamp');
-  Subject<int?> get origStreamId => has((e) => e.origStreamId, 'origStreamId');
-  Subject<int?> get newStreamId => has((e) => e.newStreamId, 'newStreamId');
-  Subject<PropagateMode?> get propagateMode => has((e) => e.propagateMode, 'propagateMode');
-  Subject<TopicName?> get origTopic => has((e) => e.origTopic, 'origTopic');
-  Subject<TopicName?> get newTopic => has((e) => e.newTopic, 'newTopic');
+  Subject<UpdateMessageMoveData?> get moveData => has((e) => e.moveData, 'moveData');
   Subject<String?> get origContent => has((e) => e.origContent, 'origContent');
   Subject<String?> get origRenderedContent => has((e) => e.origRenderedContent, 'origRenderedContent');
   Subject<String?> get content => has((e) => e.content, 'content');
@@ -60,6 +56,14 @@ extension UpdateMessageEventChecks on Subject<UpdateMessageEvent> {
   Subject<bool?> get isMeMessage => has((e) => e.isMeMessage, 'isMeMessage');
 }

+extension UpdateMessageMoveDataChecks on Subject<UpdateMessageMoveData> {
+  Subject<int> get origStreamId => has((e) => e.origStreamId, 'origStreamId');
+  Subject<int> get newStreamId => has((e) => e.newStreamId, 'newStreamId');
+  Subject<TopicName> get origTopic => has((e) => e.origTopic, 'origTopic');
+  Subject<TopicName> get newTopic => has((e) => e.newTopic, 'newTopic');
+  Subject<PropagateMode> get propagateMode => has((e) => e.propagateMode, 'propagateMode');
+}
+
 extension DeleteMessageEventChecks on Subject<DeleteMessageEvent> {
   Subject<MessageType?> get messageType => has((e) => e.messageType, 'messageType');
 }
diff --git a/test/api/model/events_test.dart b/test/api/model/events_test.dart
index 51b36350cc..7cdfa94eff 100644
--- a/test/api/model/events_test.dart
+++ b/test/api/model/events_test.dart
@@ -101,24 +101,106 @@ void main() {
       'edit_timestamp': 1718741351,
       'stream_id': eg.stream().streamId,
     };
+    final baseMoveJson = { ...baseJson,
+      'orig_subject': 'foo',
+      'propagate_mode': 'change_all',
+    };
+
+    test('smoke moveData', () {
+      check(Event.fromJson({ ...baseMoveJson,
+        'stream_id': 1,
+        'new_stream_id': 2,
+        'orig_subject': 'foo',
+        'subject': 'bar',
+        'propagate_mode': 'change_all',
+      })).isA<UpdateMessageEvent>().moveData.isNotNull()
+        ..origStreamId.equals(1)
+        ..newStreamId.equals(2)
+        ..origTopic.equals(const TopicName('foo'))
+        ..newTopic.equals(const TopicName('bar'))
+        ..propagateMode.equals(PropagateMode.changeAll);
+    });

     test('stream_id -> origStreamId', () {
-      check(Event.fromJson({ ...baseJson,
+      check(Event.fromJson({ ...baseMoveJson,
         'stream_id': 1,
         'new_stream_id': 2,
-      }) as UpdateMessageEvent)
+      })).isA<UpdateMessageEvent>().moveData.isNotNull()
         ..origStreamId.equals(1)
         ..newStreamId.equals(2);
     });

     test('orig_subject -> origTopic, subject -> newTopic', () {
-      check(Event.fromJson({ ...baseJson,
+      check(Event.fromJson({ ...baseMoveJson,
         'orig_subject': 'foo',
         'subject': 'bar',
-      }) as UpdateMessageEvent)
+      })).isA<UpdateMessageEvent>().moveData.isNotNull()
         ..origTopic.equals(const TopicName('foo'))
         ..newTopic.equals(const TopicName('bar'));
     });
+
+    test('new channel, same topic: fill in newTopic', () {
+      // The server omits 'subject' in this situation.
+      check(Event.fromJson({ ...baseMoveJson,
+        'stream_id': 1,
+        'new_stream_id': 2,
+        'orig_subject': 'foo',
+        'subject': null,
+      })).isA<UpdateMessageEvent>().moveData.isNotNull()
+        ..origTopic.equals(const TopicName('foo'))
+        ..newTopic.equals(const TopicName('foo'));
+    });
+
+    test('same channel, new topic; fill in newStreamId', () {
+      // The server omits 'new_stream_id' in this situation.
+      check(Event.fromJson({ ...baseMoveJson,
+        'stream_id': 1,
+        'new_stream_id': null,
+        'orig_subject': 'foo',
+        'subject': 'bar',
+      })).isA<UpdateMessageEvent>().moveData.isNotNull()
+        ..origStreamId.equals(1)
+        ..newStreamId.equals(1);
+    });
+
+    test('no message move', () {
+      check(Event.fromJson({ ...baseJson,
+        'orig_content': 'foo',
+        'orig_rendered_content': 'foo',
+        'content': 'bar',
+        'rendered_content': 'bar',
+      })).isA<UpdateMessageEvent>().moveData.isNull();
+    });
+
+    test('stream move but no orig_subject', () {
+      check(() => Event.fromJson({ ...baseMoveJson,
+        'stream_id': 1,
+        'new_stream_id': 2,
+        'orig_subject': null,
+      })).throws<void>();
+    });
+
+    test('move but no subject or new_stream_id', () {
+      check(() => Event.fromJson({ ...baseMoveJson,
+        'new_stream_id': null,
+        'subject': null,
+      })).throws<void>();
+    });
+
+    test('move but no orig_stream_id', () {
+      check(() => Event.fromJson({ ...baseMoveJson,
+        'stream_id': null,
+        'new_stream_id': 2,
+      })).throws<void>();
+    });
+
+    test('move but no propagate_mode', () {
+      check(() => Event.fromJson({ ...baseMoveJson,
+        'orig_subject': 'foo',
+        'subject': 'bar',
+        'propagate_mode': null,
+      })).throws<void>();
+    });
   });

   test('delete_message: require streamId and topic for stream messages', () {
diff --git a/test/api/model/model_checks.dart b/test/api/model/model_checks.dart
index 8b39b1ad57..77995466ce 100644
--- a/test/api/model/model_checks.dart
+++ b/test/api/model/model_checks.dart
@@ -53,6 +53,7 @@ extension TopicNameChecks on Subject<TopicName> {

 extension StreamMessageChecks on Subject<StreamMessage> {
   Subject<String?> get displayRecipient => has((e) => e.displayRecipient, 'displayRecipient');
+  Subject<int> get streamId => has((e) => e.streamId, 'streamId');
   Subject<TopicName> get topic => has((e) => e.topic, 'topic');
 }

diff --git a/test/example_data.dart b/test/example_data.dart
index 955f5dacfb..cfb333baec 100644
--- a/test/example_data.dart
+++ b/test/example_data.dart
@@ -655,11 +655,7 @@ UpdateMessageEvent updateMessageEditEvent(
     messageIds: [messageId],
     flags: flags ?? origMessage.flags,
     editTimestamp: editTimestamp ?? 1234567890, // TODO generate timestamp
-    origStreamId: origMessage is StreamMessage ? origMessage.streamId : null,
-    newStreamId: null,
-    propagateMode: null,
-    origTopic: null,
-    newTopic: null,
+    moveData: null,
     origContent: 'some probably-mismatched old Markdown',
     origRenderedContent: origMessage.content,
     content: 'some probably-mismatched new Markdown',
@@ -681,8 +677,6 @@ UpdateMessageEvent _updateMessageMoveEvent(
 }) {
   _checkPositive(origStreamId, 'stream ID');
   _checkPositive(newStreamId, 'stream ID');
-  assert(newTopic != origTopic
-         || (newStreamId != null && newStreamId != origStreamId));
   assert(messageIds.isNotEmpty);
   return UpdateMessageEvent(
     id: 0,
@@ -692,11 +686,13 @@ UpdateMessageEvent _updateMessageMoveEvent(
     messageIds: messageIds,
     flags: flags,
     editTimestamp: 1234567890, // TODO generate timestamp
-    origStreamId: origStreamId,
-    newStreamId: newStreamId,
-    propagateMode: propagateMode,
-    origTopic: origTopic,
-    newTopic: newTopic,
+    moveData: UpdateMessageMoveData(
+      origStreamId: origStreamId,
+      newStreamId: newStreamId ?? origStreamId,
+      origTopic: origTopic,
+      newTopic: newTopic ?? origTopic,
+      propagateMode: propagateMode,
+    ),
     origContent: origContent,
     origRenderedContent: origContent,
     content: newContent,
diff --git a/test/model/message_list_test.dart b/test/model/message_list_test.dart
index da286f3ef5..92c08ddd6b 100644
--- a/test/model/message_list_test.dart
+++ b/test/model/message_list_test.dart
@@ -825,11 +825,14 @@ void main() {
       });

       test('unrelated channel -> new channel: unaffected', () async {
+        final thirdStream = eg.stream();
         await prepareNarrow(narrow, initialMessages);
+        await store.addStream(thirdStream);
+        await store.addSubscription(eg.subscription(thirdStream));

         await store.handleEvent(eg.updateMessageEventMoveFrom(
           origMessages: otherChannelMovedMessages,
-          newStreamId: otherStream.streamId,
+          newStreamId: thirdStream.streamId,
         ));
         checkHasMessages(initialMessages);
         checkNotNotified();
diff --git a/test/model/unreads_test.dart b/test/model/unreads_test.dart
index 40e074dcaa..9a2e899ba7 100644
--- a/test/model/unreads_test.dart
+++ b/test/model/unreads_test.dart
@@ -469,6 +469,175 @@ void main() {
         }
       }
     });
+
+    group('moves', () {
+      final origChannel = eg.stream();
+      const origTopic = 'origTopic';
+      const newTopic = 'newTopic';
+
+      late List<StreamMessage> readMessages;
+      late List<StreamMessage> unreadMessages;
+
+      Future<void> prepareStore() async {
+        prepare();
+        await channelStore.addStream(origChannel);
+        await channelStore.addSubscription(eg.subscription(origChannel));
+        readMessages  = List<StreamMessage>.generate(10,
+          (_) => eg.streamMessage(stream: origChannel, topic: origTopic,
+                   flags: [MessageFlag.read]));
+        unreadMessages = List<StreamMessage>.generate(10,
+          (_) => eg.streamMessage(stream: origChannel, topic: origTopic));
+      }
+
+      List<StreamMessage> copyMessagesWith(Iterable<StreamMessage> messages, {
+        ZulipStream? newChannel,
+        String? newTopic,
+      }) {
+        assert(newChannel != null || newTopic != null);
+        return messages.map((message) => StreamMessage.fromJson(
+          message.toJson()
+            ..['stream_id'] = newChannel?.streamId ?? message.streamId
+            ..['subject'] = newTopic ?? message.topic
+        )).toList();
+      }
+
+      test('moved messages = unread messages', () async {
+        await prepareStore();
+        final newChannel = eg.stream();
+        await channelStore.addStream(newChannel);
+        await channelStore.addSubscription(eg.subscription(newChannel));
+        fillWithMessages(unreadMessages);
+        final originalMessageIds =
+          model.streams[origChannel.streamId]![TopicName(origTopic)]!;
+
+        model.handleUpdateMessageEvent(eg.updateMessageEventMoveFrom(
+          origMessages: unreadMessages,
+          newStreamId: newChannel.streamId,
+          newTopicStr: newTopic));
+        checkNotifiedOnce();
+        checkMatchesMessages(copyMessagesWith(unreadMessages,
+          newChannel: newChannel, newTopic: newTopic));
+        final newMessageIds =
+          model.streams[newChannel.streamId]![TopicName(newTopic)]!;
+        // Check we successfully avoided making a copy of the list.
+        check(originalMessageIds).identicalTo(newMessageIds);
+      });
+
+      test('moved messages ⊂ read messages', () async {
+        await prepareStore();
+        final messagesToMove = readMessages.take(2).toList();
+        fillWithMessages(unreadMessages + readMessages);
+
+        model.handleUpdateMessageEvent(eg.updateMessageEventMoveFrom(
+          origMessages: messagesToMove,
+          newTopicStr: newTopic));
+        checkNotNotified();
+        checkMatchesMessages(unreadMessages);
+      });
+
+      test('moved messages ⊂ unread messages', () async {
+        await prepareStore();
+        final messagesToMove = unreadMessages.take(2).toList();
+        fillWithMessages(unreadMessages + readMessages);
+
+        model.handleUpdateMessageEvent(eg.updateMessageEventMoveFrom(
+          origMessages: messagesToMove,
+          newTopicStr: newTopic));
+        checkNotifiedOnce();
+        checkMatchesMessages([
+          ...copyMessagesWith(messagesToMove, newTopic: newTopic),
+          ...unreadMessages.skip(2),
+        ]);
+      });
+
+      test('moved messages ∩ unread messages ≠ Ø, moved messages ∩ read messages ≠ Ø, moved messages ⊅ unread messages', () async {
+        await prepareStore();
+        final messagesToMove = [unreadMessages.first, readMessages.first];
+        fillWithMessages(unreadMessages + readMessages);
+
+        model.handleUpdateMessageEvent(eg.updateMessageEventMoveFrom(
+          origMessages: messagesToMove,
+          newTopicStr: newTopic));
+        checkNotifiedOnce();
+        checkMatchesMessages([
+          ...copyMessagesWith(unreadMessages.take(1), newTopic: newTopic),
+          ...unreadMessages.skip(1),
+        ]);
+      });
+
+      test('moved messages ⊃ unread messages', () async {
+        await prepareStore();
+        final messagesToMove = unreadMessages + readMessages.take(2).toList();
+        fillWithMessages(unreadMessages + readMessages);
+        final originalMessageIds =
+          model.streams[origChannel.streamId]![TopicName(origTopic)]!;
+
+        model.handleUpdateMessageEvent(eg.updateMessageEventMoveFrom(
+          origMessages: messagesToMove,
+          newTopicStr: newTopic));
+        checkNotifiedOnce();
+        checkMatchesMessages(copyMessagesWith(unreadMessages, newTopic: newTopic));
+        final newMessageIds =
+          model.streams[origChannel.streamId]![TopicName(newTopic)]!;
+        // Check we successfully avoided making a copy of the list.
+        check(originalMessageIds).identicalTo(newMessageIds);
+      });
+
+      test('moving to unsubscribed channels drops the unreads', () async {
+        await prepareStore();
+        final unsubscribedChannel = eg.stream();
+        await channelStore.addStream(unsubscribedChannel);
+        assert(!channelStore.subscriptions.containsKey(
+          unsubscribedChannel.streamId));
+        fillWithMessages(unreadMessages);
+
+        model.handleUpdateMessageEvent(eg.updateMessageEventMoveFrom(
+          origMessages: unreadMessages,
+          newStreamId: unsubscribedChannel.streamId));
+        checkNotifiedOnce();
+        checkMatchesMessages([]);
+      });
+
+      test('tolerates unsorted messages', () async {
+        await prepareStore();
+        final unreadMessages = List.generate(10, (i) =>
+          eg.streamMessage(
+            id: 1000 - i, stream: origChannel, topic: origTopic));
+        fillWithMessages(unreadMessages);
+
+        model.handleUpdateMessageEvent(eg.updateMessageEventMoveFrom(
+          origMessages: unreadMessages,
+          newTopicStr: newTopic));
+        checkNotifiedOnce();
+        checkMatchesMessages(copyMessagesWith(unreadMessages, newTopic: newTopic));
+      });
+
+      test('tolerates unreads unknown to the model', () async {
+        await prepareStore();
+        fillWithMessages(unreadMessages);
+
+        final unknownChannel = eg.stream();
+        assert(!channelStore.streams.containsKey(unknownChannel.streamId));
+        final unknownUnreadMessage = eg.streamMessage(
+          stream: unknownChannel, topic: origTopic);
+
+        model.handleUpdateMessageEvent(eg.updateMessageEventMoveFrom(
+          origMessages: [unknownUnreadMessage],
+          newTopicStr: newTopic));
+        checkNotNotified();
+        checkMatchesMessages(unreadMessages);
+      });
+
+      test('message edit but no move', () async {
+        await prepareStore();
+        fillWithMessages(unreadMessages);
+
+        model.handleUpdateMessageEvent(eg.updateMessageEditEvent(
+          unreadMessages.first));
+        checkNotNotified();
+        checkMatchesMessages(unreadMessages);
+      });
+    });
   });