From 0247a03f3f26eb69a5715a8ba8e02fd6f2c26ee6 Mon Sep 17 00:00:00 2001
From: Henry Su <henrysu4707@gmail.com>
Date: Thu, 10 Sep 2026 11:12:59 -0500
Subject: [PATCH] fix(realtime): process every key in a presence diff's leaves

_sync_diff broke out of the leaves loop when a key was not present in the
local state, so every remaining key in that diff was skipped: those
presences stayed in presence.state forever and on_presence_leave never
fired for them. Since leaves comes straight off the wire, a server diff
listing an untracked key before a tracked one is enough to trigger it.

Skip only the untracked key, matching Phoenix's presence.js.

Fixes #1630
---
 src/realtime/src/realtime/_async/presence.py |  2 +-
 src/realtime/tests/test_presence.py          | 29 ++++++++++++++++++++
 2 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/src/realtime/src/realtime/_async/presence.py b/src/realtime/src/realtime/_async/presence.py
index ad3767029..fc36550b2 100644
--- a/src/realtime/src/realtime/_async/presence.py
+++ b/src/realtime/src/realtime/_async/presence.py
@@ -117,7 +117,7 @@ def _sync_diff(
             current_presences = self.state.get(key, [])
 
             if len(current_presences) == 0:
-                break
+                continue
 
             presence_refs_to_remove = {
                 presence.get("presence_ref") for presence in left_presences
diff --git a/src/realtime/tests/test_presence.py b/src/realtime/tests/test_presence.py
index 0d4c587da..c95a4c25b 100644
--- a/src/realtime/tests/test_presence.py
+++ b/src/realtime/tests/test_presence.py
@@ -273,3 +273,32 @@ async def test_resubscribe_on_presence_callback_addition() -> None:
 
     # Verify resubscribe was called
     assert channel._resubscribe.call_count == 1
+
+
+def test_sync_diff_processes_every_leave_key() -> None:
+    """Leaves for a key the client never tracked must not skip the remaining keys."""
+    presence = AsyncRealtimePresence()
+    leave_events: List[Tuple[str, List[Dict], List[Presence]]] = []
+    presence.on_leave(
+        lambda key, current, left: leave_events.append((key, current, left))
+    )
+
+    state: RawPresenceState = {
+        "bob": {"metas": [{"phx_ref": "ref-bob"}]},
+    }
+    presence._on_state_event(state)
+    assert list(presence.state) == ["bob"]
+
+    # The server reports both an untracked key and a tracked one leaving.
+    presence._on_diff_event(
+        {
+            "joins": {},
+            "leaves": {
+                "alice": {"metas": [{"phx_ref": "ref-alice"}]},
+                "bob": {"metas": [{"phx_ref": "ref-bob"}]},
+            },
+        }
+    )
+
+    assert presence.state == {}
+    assert [key for key, _, _ in leave_events] == ["bob"]