Moved thought reverts to previous parent

[raineorshine]

Platform: Chrome
Seed: 1m
Initial replication

Steps to Reproduce

Perform the following steps during the initial replication:

- a
  - b
    - c

1. Drag c into to a.

Thoughts will look like this:

- a
  - b
  - c

2. Now quickly drag c back into b:

- a
  - b
    - c

Current Behavior

c becomes a child of b, then quickly reverts to being a child of a:

Screen.Recording.2025-05-01.at.11.33.42.AM.mov

Expected Behavior

c should stay a child of b.

[obezzad]

I tried to reproduce this. It seems more of a (very slow) flickering on my side as the second change eventually applies.

PowerSync pushes all uploads then performs a state machine replication; making the client replay the server state blindly after pushing all pending queue. This can make the client go back few steps as the sync is catching up (see image below. Source).

However, since the operations are getting applied synchronously server-side with Postgres functions, they are sent back quite fast and the flickering shouldn't be noticeable; only possible bottleneck is the change capture (CDC) at PowerSync service (which usually takes few millis) and client-side DB read (which should be in few ms as well, but partial sync is still quite slow at DB reads as explained in #2 (comment), which is probably the cause here as well). I recognize it gets even more confusing as the server overrides the _pending flag when the first change is finalized (making it look like the second one is not coming).

An upcoming feature powersync-ja/powersync-js#593 can help us prune operations client-side without giving up on consistency to mitigate this issue. We can also use getNextCrudBatch from now, although with slightly more complexity.

However, if my observations about the second change eventually applied is always the case, I believe addressing this flickering before the local DB querying performance issue may lead to over-engineering. The querying issue, especially at partial sync, is the root cause of all open issues so far. I'm working hard on a solution for that.