diff --git a/mobile/lib/features/activity/activity_page.dart b/mobile/lib/features/activity/activity_page.dart
index afbea12ab7..b0f8225caa 100644
--- a/mobile/lib/features/activity/activity_page.dart
+++ b/mobile/lib/features/activity/activity_page.dart
@@ -1,7 +1,7 @@
 import 'package:buzz/shared/widgets/feedback_snackbar.dart';
 
 import 'dart:async';
-import 'dart:math' show max;
+import 'dart:math' show max, min;
 
 import '../../shared/widgets/ios_message_create_button.dart';
 import '../../shared/widgets/ios_glass_navigation_button.dart';
@@ -213,18 +213,52 @@ class ActivityPage extends HookConsumerWidget {
       return null;
     }, [preloadPubkeysKey]);
 
+    // A filtered row holds only part of its conversation. Advance its synced
+    // thread or channel marker only up to the first unread event the filter
+    // hides, and give shown events past that point local per-message marks.
+    void markPartialRowRead(
+      InboxItem item,
+      String channelId, {
+      required bool isDmRow,
+    }) {
+      final notifier = ref.read(readStateProvider.notifier);
+      final shown = {
+        for (final e in [item.item, ...item.groupItems]) e.id: e,
+      };
+      final rootId = isDmRow ? null : item.threadRootId;
+      bool inContext(FeedItem e) =>
+          e.channelId == channelId &&
+          (rootId == null ||
+              (isThreadReply(e.tags) &&
+                  threadReferenceOf(e.tags).rootId == rootId));
+      var cap = item.latestActivityAt;
+      for (final e in allItems.expand((i) => i.groupItems)) {
+        if (shown.containsKey(e.id) ||
+            !inContext(e) ||
+            isInboxEventRead(e, markerOf: markerOf)) {
+          continue;
+        }
+        cap = min(cap, e.createdAt - 1);
+      }
+      notifier.markContextRead(
+        rootId == null ? channelId : threadContextKey(rootId),
+        cap,
+      );
+      for (final e in shown.values) {
+        if (inContext(e) && e.createdAt <= cap) continue;
+        notifier.markContextRead(msgContextKey(e.id), e.createdAt);
+      }
+    }
+
     void markItemRead(InboxItem item) {
       final notifier = ref.read(readStateProvider.notifier);
       ref
           .read(inboxLocalStateProvider.notifier)
           .clearUnread(groupedInboxItemIds(item));
-      // A filtered row holds only part of its conversation, so channel and
-      // thread markers would also read events this view hides.
       final isDmRow = item.conversationId.startsWith('dm:');
-      if (rowsArePartial && item.item.channelId != null) {
-        for (final e in {item.item, ...item.groupItems}) {
-          notifier.markContextRead(msgContextKey(e.id), e.createdAt);
-        }
+      final partialChannelId = rowsArePartial ? item.item.channelId : null;
+      if (partialChannelId != null) {
+        markPartialRowRead(item, partialChannelId, isDmRow: isDmRow);
         return;
       }
       // A DM row can group replies under several thread roots; its channel
diff --git a/mobile/test/features/activity/activity_page_test.dart b/mobile/test/features/activity/activity_page_test.dart
index eb4711f912..e1976ed0cb 100644
--- a/mobile/test/features/activity/activity_page_test.dart
+++ b/mobile/test/features/activity/activity_page_test.dart
@@ -18,6 +18,7 @@ import 'package:buzz/features/channels/message_content.dart';
 import 'package:buzz/features/channels/channels_provider.dart';
 import 'package:buzz/shared/identity_names/identity_names.dart';
 import 'package:buzz/shared/mentions/agent_identity_provider.dart';
+import 'package:buzz/shared/read_state/read_state_format.dart';
 import 'package:buzz/shared/read_state/read_state_provider.dart';
 import 'package:buzz/shared/profile/user_cache_provider.dart';
 import 'package:buzz/shared/profile/user_profile.dart';
@@ -930,6 +931,21 @@ void main() {
 
       await chooseSender('Anyone');
       expect(find.byKey(const ValueKey('inbox-unread-dot-p1')), findsOneWidget);
+      int? threadMarker() => ProviderScope.containerOf(
+        tester.element(find.byType(ActivityPage)),
+      ).read(readStateProvider).contexts[threadContextKey(threadIdRoot1)];
+      // The synced marker stops short of the hidden agent reply.
+      expect(threadMarker(), now - 301);
+
+      await chooseSender('Agents');
+      await tester.drag(
+        find.byKey(const ValueKey('inbox-row-g1')),
+        const Offset(-500, 0),
+      );
+      await tester.pumpAndSettle();
+      expect(threadMarker(), now - 300);
+      await chooseSender('Anyone');
+      expect(find.byKey(const ValueKey('inbox-unread-dot-p1')), findsNothing);
     },
   );