From 14a30ca0648477def3fae0dab6bf1142cf4a4e37 Mon Sep 17 00:00:00 2001
From: Paul Chobert <paul@chobert.fr>
Date: Tue, 27 Oct 2026 17:20:41 +0100
Subject: [PATCH] Strip the user's real membership from announcement room
 timelines

---
 synapse/rest/client/sync.py    | 8 ++++++++
 tests/rest/client/test_sync.py | 9 +++++++++
 2 files changed, 17 insertions(+)

diff --git a/synapse/rest/client/sync.py b/synapse/rest/client/sync.py
index cd53bb5e6f..0b840d9e44 100644
--- a/synapse/rest/client/sync.py
+++ b/synapse/rest/client/sync.py
@@ -612,6 +612,14 @@ class SyncRestServlet(RestServlet):
                 for key, event in state_dict.items()
                 if key != (EventTypes.Member, user_id)
             }
+            timeline_events = [
+                filtered_event
+                for filtered_event in timeline_events
+                if not (
+                    filtered_event.event.type == EventTypes.Member
+                    and filtered_event.event.state_key == user_id
+                )
+            ]
 
         state_events = state_dict.values()
 
diff --git a/tests/rest/client/test_sync.py b/tests/rest/client/test_sync.py
index 29579c820a..3cfb3bcc6b 100644
--- a/tests/rest/client/test_sync.py
+++ b/tests/rest/client/test_sync.py
@@ -2361,6 +2361,15 @@ class ServerAnnouncementsSyncTestCase(unittest.HomeserverTestCase):
         )
         self.assertEqual(self._timeline_ids(room), [event_id])
 
+    def test_initial_sync_after_leaving(self) -> None:
+        """A former member who left on their own gets a single, synthetic join
+        on initial sync, even when their real leave is among the recent events.
+        """
+        self.helper.leave(self.room_id, self.former, tok=self.former_tok)
+        self._announce("hello")
+
+        self._assert_delivered(self._sync(self.former_tok), self.former)
+
     def test_lazy_loaded_members(self) -> None:
         """The synthetic join is sent when members are lazy-loaded."""
         self._announce("hello")