Fix newly_left rooms not appearing if we returned early (Sliding Sync) (#17301)

Fix `newly_left` rooms not appearing if we returned early when `membership_snapshot_token.is_before_or_eq(to_token.room_key)`. 

Introduced in #17187 (part of Sliding Sync)

The tests didn't catch it because they had a small typo in it `room_id1` vs `room_id2`.

Found while working on #17293

Author: MadLittleMods

changelog.d/17301.bugfix

@@ -0,0 +1 @@
+Add initial implementation of an experimental [MSC3575](https://github.com/matrix-org/matrix-spec-proposals/pull/3575) Sliding Sync `/sync` endpoint.

synapse/handlers/sliding_sync.py

@@ -275,12 +275,6 @@ async def get_sync_room_ids_for_user(
             instance_map=immutabledict(instance_to_max_stream_ordering_map),
         )
 
-        # If our `to_token` is already the same or ahead of the latest room membership
-        # for the user, we can just straight-up return the room list (nothing has
-        # changed)
-        if membership_snapshot_token.is_before_or_eq(to_token.room_key):
-            return sync_room_id_set
-
         # Since we fetched the users room list at some point in time after the from/to
         # tokens, we need to revert/rewind some membership changes to match the point in
         # time of the `to_token`. In particular, we need to make these fixups:
@@ -300,14 +294,20 @@ async def get_sync_room_ids_for_user(
 
         # 1) Fetch membership changes that fall in the range from `to_token` up to
         # `membership_snapshot_token`
-        membership_change_events_after_to_token = (
-            await self.store.get_membership_changes_for_user(
-                user_id,
-                from_key=to_token.room_key,
-                to_key=membership_snapshot_token,
-                excluded_rooms=self.rooms_to_exclude_globally,
+        #
+        # If our `to_token` is already the same or ahead of the latest room membership
+        # for the user, we don't need to do any "2)" fix-ups and can just straight-up
+        # use the room list from the snapshot as a base (nothing has changed)
+        membership_change_events_after_to_token = []
+        if not membership_snapshot_token.is_before_or_eq(to_token.room_key):
+            membership_change_events_after_to_token = (
+                await self.store.get_membership_changes_for_user(
+                    user_id,
+                    from_key=to_token.room_key,
+                    to_key=membership_snapshot_token,
+                    excluded_rooms=self.rooms_to_exclude_globally,
+                )
             )
-        )
 
         # 1) Assemble a list of the last membership events in some given ranges. Someone
         # could have left and joined multiple times during the given range but we only

tests/handlers/test_sliding_sync.py

@@ -326,7 +326,7 @@ def test_only_newly_left_rooms_show_up(self) -> None:
 
         # Leave during the from_token/to_token range (newly_left)
         room_id2 = self.helper.create_room_as(user1_id, tok=user1_tok)
-        self.helper.leave(room_id1, user1_id, tok=user1_tok)
+        self.helper.leave(room_id2, user1_id, tok=user1_tok)
 
         after_room2_token = self.event_sources.get_current_token()