fix(#343): keep full message history; window only in memory

The persisted store was being overwritten with the windowed in-memory list, so
any channel or DM with more than _messageWindowSize (200) messages lost
everything older than the newest 200 on the first save after load. Observed
live: Public went 232 -> 201 in one session on a build that already had the
#333 fix, so this was not the race - it was windowing truncating the store.
This is the slow-erosion cause behind the whole 566 -> 231 -> 206 -> 201
history.

saveChannelMessages / saveMessages now MERGE into the persisted set instead of
overwriting: upsert by message identity so older persisted messages are kept,
new ones added, and the in-memory copy wins for edits/reactions/status. If the
existing history fails to decode, the save aborts loudly rather than merging
into an empty base and truncating (SAFELANE 6).

Deletion is now an explicit path - removeChannelMessage / removeMessage - since
the app deletes individual messages. Routing delete through the merging save
would resurrect them; the connector's deleteChannelMessage / deleteMessage now
call the explicit remove.

Windowing stays for display and memory; it no longer dictates what is stored.

Tests: 250-message history survives a 200-window save; new message appends;
edit is captured not duplicated; delete does not resurrect. 508 pass, analyze
and format clean.

Cost: a save now re-reads and re-encodes the channel/contact history. Cheap
with drift's per-key writes (#335); a future append-only schema removes even
that.

lib/connector/meshcore_connector.dart

@@ -699,7 +699,8 @@ class MeshCoreConnector extends ChangeNotifier {
     if (messages == null) return;
     final removed = messages.remove(message);
     if (!removed) return;
-    await _messageStore.saveMessages(contactKeyHex, messages);
+    // Explicit delete: saveMessages now merges and never removes (#343).
+    await _messageStore.removeMessage(contactKeyHex, message);
     notifyListeners();
   }
 
@@ -833,7 +834,10 @@ class MeshCoreConnector extends ChangeNotifier {
     if (messages == null) return;
     final removed = messages.remove(message);
     if (!removed) return;
-    await _channelMessageStore.saveChannelMessages(channelIndex, messages);
+    // Explicit delete path: saveChannelMessages now MERGES and never removes
+    // (#343), so deletion must go through removeChannelMessage or the message
+    // would be resurrected on the next save.
+    await _channelMessageStore.removeChannelMessage(channelIndex, message);
     notifyListeners();
   }
 

lib/storage/channel_message_store.dart

@@ -65,10 +65,83 @@ class ChannelMessageStore {
       );
       return;
     }
-    final jsonList = messages.map((msg) => _messageToJson(msg)).toList();
+    // Merge into the persisted full history rather than overwriting it. The
+    // in-memory list is windowed to the most recent N for memory, so a plain
+    // overwrite would truncate the store to N and erode old history (#343).
+    // Upsert by identity: keep older persisted messages, add new ones, and let
+    // the in-memory copy win so edits/reactions/status updates are captured.
+    // Deletion has its own path (removeChannelMessage) so this never
+    // resurrects a message the user deleted.
+    final key = _storageKey(channelIndex);
+    final byKey = <String, ChannelMessage>{};
+
+    final existing = await BlobStore.instance.readWithPrefsFallback(key);
+    if (existing != null && existing.isNotEmpty) {
+      try {
+        for (final e in jsonDecode(existing) as List<dynamic>) {
+          final m = _messageFromJson(e as Map<String, dynamic>);
+          byKey[_mergeKey(m)] = m;
+        }
+      } catch (e) {
+        // SAFELANE 6: a decode failure here must be loud, not silently drop the
+        // persisted history by merging into an empty base.
+        appLogger.error(
+          'Failed to decode existing channel $channelIndex history before '
+          'merge; aborting save to avoid truncation: $e',
+          tag: 'Storage',
+        );
+        return;
+      }
+    }
+    for (final m in messages) {
+      byKey[_mergeKey(m)] = m;
+    }
+
+    final merged = byKey.values.toList()
+      ..sort((a, b) => a.timestamp.compareTo(b.timestamp));
+    await BlobStore.instance.write(
+      key,
+      jsonEncode(merged.map(_messageToJson).toList()),
+    );
+  }
+
+  /// Stable identity for merge/dedupe. messageId when present, else a composite
+  /// that distinguishes distinct messages that share no id.
+  String _mergeKey(ChannelMessage m) {
+    if (m.messageId.isNotEmpty) return 'id:${m.messageId}';
+    return 'x:${m.packetHash ?? ''}:${m.timestamp.millisecondsSinceEpoch}:${m.text}';
+  }
+
+  /// Removes a single message from the persisted history. The explicit delete
+  /// path (#343): save merges and never removes, so deletion cannot go through
+  /// save.
+  Future<void> removeChannelMessage(
+    int channelIndex,
+    ChannelMessage message,
+  ) async {
+    if (publicKeyHex.isEmpty) return;
+    final key = _storageKey(channelIndex);
+    final existing = await BlobStore.instance.readWithPrefsFallback(key);
+    if (existing == null || existing.isEmpty) return;
+
+    final List<dynamic> raw;
+    try {
+      raw = jsonDecode(existing) as List<dynamic>;
+    } catch (e) {
+      appLogger.error(
+        'Failed to decode channel $channelIndex history for delete: $e',
+        tag: 'Storage',
+      );
+      return;
+    }
+    final target = _mergeKey(message);
+    final kept = raw
+        .map((e) => _messageFromJson(e as Map<String, dynamic>))
+        .where((m) => _mergeKey(m) != target)
+        .toList();
     await BlobStore.instance.write(
-      _storageKey(channelIndex),
-      jsonEncode(jsonList),
+      key,
+      jsonEncode(kept.map(_messageToJson).toList()),
     );
   }
 

lib/storage/drift/blob_store.dart

@@ -1,3 +1,5 @@
+import 'package:flutter/foundation.dart';
+
 import '../../utils/app_logger.dart';
 import '../prefs_manager.dart';
 import 'offband_database.dart';
@@ -19,10 +21,19 @@ class BlobStore {
   final OffbandDatabase _db;
 
   static OffbandDatabase? _sharedDb;
+  static BlobStore? _override;
 
   /// Process-wide instance. The database must be opened once; opening it twice
   /// is an error on the web backends.
-  static BlobStore get instance => BlobStore(_sharedDb ??= OffbandDatabase());
+  static BlobStore get instance =>
+      _override ?? BlobStore(_sharedDb ??= OffbandDatabase());
+
+  /// Test seam: point the singleton at an in-memory database.
+  @visibleForTesting
+  static void overrideForTest(BlobStore store) => _override = store;
+
+  @visibleForTesting
+  static void clearTestOverride() => _override = null;
 
   /// Key families that hold bulk data. Everything else stays in
   /// SharedPreferences, which is what it is for.

lib/storage/message_store.dart

@@ -24,9 +24,71 @@ class MessageStore {
       appLogger.warn('Public key hex is not set. Cannot save messages.');
       return;
     }
+    // Merge into the persisted full history rather than overwriting it (#343).
+    // The in-memory list is windowed for memory, so a plain overwrite would
+    // truncate the store. Upsert by identity; deletion is explicit
+    // (removeMessage).
     final key = '$keyFor$contactKeyHex';
-    final jsonList = messages.map(_messageToJson).toList();
-    await BlobStore.instance.write(key, jsonEncode(jsonList));
+    final byKey = <String, Message>{};
+
+    final existing = await BlobStore.instance.readWithPrefsFallback(key);
+    if (existing != null && existing.isNotEmpty) {
+      try {
+        for (final e in jsonDecode(existing) as List<dynamic>) {
+          final m = _messageFromJson(e as Map<String, dynamic>);
+          byKey[_mergeKey(m)] = m;
+        }
+      } catch (e) {
+        appLogger.error(
+          'Failed to decode existing DM history before merge; aborting save '
+          'to avoid truncation: $e',
+          tag: 'Storage',
+        );
+        return;
+      }
+    }
+    for (final m in messages) {
+      byKey[_mergeKey(m)] = m;
+    }
+
+    final merged = byKey.values.toList()
+      ..sort((a, b) => a.timestamp.compareTo(b.timestamp));
+    await BlobStore.instance.write(
+      key,
+      jsonEncode(merged.map(_messageToJson).toList()),
+    );
+  }
+
+  String _mergeKey(Message m) {
+    if (m.messageId.isNotEmpty) return 'id:${m.messageId}';
+    return 'x:${m.senderKeyHex}:${m.timestamp.millisecondsSinceEpoch}:${m.text}';
+  }
+
+  /// Explicit delete path (#343): save merges and never removes.
+  Future<void> removeMessage(String contactKeyHex, Message message) async {
+    if (publicKeyHex.isEmpty) return;
+    final key = '$keyFor$contactKeyHex';
+    final existing = await BlobStore.instance.readWithPrefsFallback(key);
+    if (existing == null || existing.isEmpty) return;
+    final List<dynamic> raw;
+    try {
+      raw = jsonDecode(existing) as List<dynamic>;
+    } catch (e) {
+      appLogger.error(
+        'Failed to decode DM history for delete: $e',
+        tag: 'Storage',
+      );
+      return;
+    }
+    final target = _mergeKey(message);
+    final kept = raw
+        .map((e) => _messageFromJson(e as Map<String, dynamic>))
+        .where((m) => _mergeKey(m) != target)
+        .toList();
+    await BlobStore.instance.write(
+      key,
+      jsonEncode(kept.map(_messageToJson).toList()),
+    );
   }
 
   Future<List<Message>> loadMessages(String contactKeyHex) async {

test/storage/channel_message_merge_test.dart

@@ -0,0 +1,103 @@
+import 'package:drift/native.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:meshcore_open/models/channel_message.dart';
+import 'package:meshcore_open/storage/channel_message_store.dart';
+import 'package:meshcore_open/storage/drift/blob_store.dart';
+import 'package:meshcore_open/storage/drift/offband_database.dart';
+import 'package:meshcore_open/storage/prefs_manager.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+/// #343: saving a windowed in-memory list must not truncate the persisted
+/// history. The store keeps full history; deletion is explicit.
+void main() {
+  late OffbandDatabase db;
+  late ChannelMessageStore store;
+
+  setUp(() async {
+    SharedPreferences.setMockInitialValues({});
+    PrefsManager.reset();
+    await PrefsManager.initialize();
+    db = OffbandDatabase(NativeDatabase.memory());
+    BlobStore.overrideForTest(BlobStore(db));
+    store = ChannelMessageStore();
+    store.setPublicKeyHex = 'a' * 20;
+    // No PSK resolver -> uses the slot-index key, which is fine for the test.
+  });
+
+  tearDown(() async {
+    await db.close();
+    BlobStore.clearTestOverride();
+  });
+
+  ChannelMessage msg(int i) => ChannelMessage(
+    senderName: 's',
+    text: 'm$i',
+    timestamp: DateTime.fromMillisecondsSinceEpoch(1000 + i),
+    isOutgoing: false,
+    channelIndex: 0,
+    messageId: 'id$i',
+  );
+
+  test('saving a 200-window does not truncate a 250-message history', () async {
+    // Seed 250 messages, as if a full history were persisted.
+    await store.saveChannelMessages(0, [for (var i = 0; i < 250; i++) msg(i)]);
+    expect((await store.loadChannelMessages(0)).length, 250);
+
+    // The app now saves only the most-recent 200 (its in-memory window).
+    final window = [for (var i = 50; i < 250; i++) msg(i)];
+    await store.saveChannelMessages(0, window);
+
+    // Full history must survive: the older 50 are still there.
+    final all = await store.loadChannelMessages(0);
+    expect(all.length, 250, reason: 'the older 50 must not be dropped');
+    expect(all.first.text, 'm0');
+    expect(all.last.text, 'm249');
+  });
+
+  test('a new message appends without dropping old history', () async {
+    await store.saveChannelMessages(0, [for (var i = 0; i < 10; i++) msg(i)]);
+    await store.saveChannelMessages(0, [msg(10)]);
+    final all = await store.loadChannelMessages(0);
+    expect(all.length, 11);
+    expect(all.last.text, 'm10');
+  });
+
+  test('an edit to a message is captured, not duplicated', () async {
+    await store.saveChannelMessages(0, [msg(1)]);
+    final edited = ChannelMessage(
+      senderName: 's',
+      text: 'm1',
+      timestamp: DateTime.fromMillisecondsSinceEpoch(1001),
+      isOutgoing: false,
+      channelIndex: 0,
+      messageId: 'id1',
+      reactions: {'thumbsup': 2},
+    );
+    await store.saveChannelMessages(0, [edited]);
+    final all = await store.loadChannelMessages(0);
+    expect(all, hasLength(1));
+    expect(all.single.reactions['thumbsup'], 2);
+  });
+
+  test('deleting a message does NOT resurrect it on the next save', () async {
+    await store.saveChannelMessages(0, [for (var i = 0; i < 5; i++) msg(i)]);
+
+    // Delete via the explicit path.
+    await store.removeChannelMessage(0, msg(2));
+    expect(
+      (await store.loadChannelMessages(0)).map((m) => m.text),
+      isNot(contains('m2')),
+    );
+
+    // A later save of the remaining in-memory list must not bring it back.
+    final remaining = [
+      for (var i = 0; i < 5; i++)
+        if (i != 2) msg(i),
+    ];
+    await store.saveChannelMessages(0, remaining);
+
+    final all = await store.loadChannelMessages(0);
+    expect(all.map((m) => m.text), isNot(contains('m2')));
+    expect(all, hasLength(4));
+  });
+}