From 59b1652f4a4e289099ea4750e6c1ca79339e5f96 Mon Sep 17 00:00:00 2001
From: Erwan Leboucher <erwanleboucher@gmail.com>
Date: Thu, 10 Sep 2026 17:11:33 +0200
Subject: [PATCH] fix(sync): Keep sync positions of rooms outside a list's
 window

---
 .../sliding-sync-window-positions.bugfix.md   |  1 +
 src/service/sync/mod.rs                       | 45 ++++++++++++++-----
 2 files changed, 35 insertions(+), 11 deletions(-)
 create mode 100644 changelog.d/sliding-sync-window-positions.bugfix.md

diff --git a/changelog.d/sliding-sync-window-positions.bugfix.md b/changelog.d/sliding-sync-window-positions.bugfix.md
new file mode 100644
index 000000000..c2207f8db
--- /dev/null
+++ b/changelog.d/sliding-sync-window-positions.bugfix.md
@@ -0,0 +1 @@
+Fixed simplified Sliding Sync re-sending rooms in full after a list's window shrinks and grows again. Contributed by @eleboucher.
diff --git a/src/service/sync/mod.rs b/src/service/sync/mod.rs
index cfe8510ff..5b50b38de 100644
--- a/src/service/sync/mod.rs
+++ b/src/service/sync/mod.rs
@@ -52,6 +52,17 @@ fn acknowledge(&mut self, pos: u64) {
 			self.acknowledged_pos = pos;
 		}
 	}
+
+	fn stage_pending(&mut self, pos: u64, known_rooms: BTreeMap<String, BTreeSet<OwnedRoomId>>) {
+		let mut pending_rooms = self.known_rooms.clone();
+		for (list_id, rooms) in known_rooms {
+			let list = pending_rooms.entry(list_id).or_default();
+			for room_id in rooms {
+				list.insert(room_id, pos);
+			}
+		}
+		self.pending.insert(pos, pending_rooms);
+	}
 }
 
 type DbConnections<K, V> = SyncMutex<BTreeMap<K, V>>;
@@ -285,17 +296,7 @@ pub fn store_snake_sync_pending(
 		let cached = &mut cached.lock();
 		drop(cache);
 
-		let mut pending_rooms = cached.known_rooms.clone();
-		for (list_id, rooms) in known_rooms {
-			let list = pending_rooms.entry(list_id).or_default();
-			for last_seen in list.values_mut() {
-				*last_seen = 0;
-			}
-			for room_id in rooms {
-				list.insert(room_id, pos);
-			}
-		}
-		cached.pending.insert(pos, pending_rooms);
+		cached.stage_pending(pos, known_rooms);
 	}
 
 	pub fn update_snake_sync_subscriptions(
@@ -432,6 +433,28 @@ fn pending_rooms_are_committed_only_when_the_response_pos_is_acknowledged() {
 		assert_eq!(cache.known_rooms["list"].len(), 1);
 	}
 
+	#[test]
+	fn rooms_outside_the_served_ranges_keep_their_position() {
+		let windowed = OwnedRoomId::try_from("!windowed:example.com").unwrap();
+		let dropped = OwnedRoomId::try_from("!dropped:example.com").unwrap();
+		let mut cache = SnakeSyncCache::default();
+		let stage = |cache: &mut SnakeSyncCache, pos, rooms: BTreeSet<OwnedRoomId>| {
+			cache.stage_pending(pos, BTreeMap::from([("all_rooms".to_owned(), rooms)]));
+			cache.acknowledge(pos);
+		};
+
+		stage(&mut cache, 100, BTreeSet::from([windowed.clone(), dropped.clone()]));
+		stage(&mut cache, 200, BTreeSet::from([windowed.clone()]));
+
+		assert_eq!(cache.known_rooms["all_rooms"][&windowed], 200);
+		assert_eq!(cache.known_rooms["all_rooms"][&dropped], 100);
+
+		stage(&mut cache, 300, BTreeSet::from([windowed.clone(), dropped.clone()]));
+
+		assert_eq!(cache.known_rooms["all_rooms"][&windowed], 300);
+		assert_eq!(cache.known_rooms["all_rooms"][&dropped], 300);
+	}
+
 	#[test]
 	fn overlapping_responses_accept_either_response_position() {
 		let mut cache = SnakeSyncCache {