There is a lifecycle issue in the sample that would work against the snapshot-plus-delta design: getKey is an inline function in LiveActivityContainer, while the hook includes getKey in its effect dependencies. Each setData update rerenders the component, creates a new function and can close and reopen the socket.

I would give that key extractor a stable identity and add a test asserting that several incoming updates keep one connection open. Otherwise a healthy event stream can produce its own reconnect churn, even after removing every router.refresh call. That is separate from deduplicating the initial snapshot.