There's no need to use messageStoreAfterReassignment. The only property of MessageStore that is not enumerated (and as a result, overriden) below is currentAsOf, which is not touched by reassignMessagesToRealizedThreads

That's a fair concern. I initially wanted to remove messageStore from the return of reassignMessagesToRealizedThreads entirely, but I didn't have time to test changes to updateMessageStoreWithLatestThreadInfos.

However, I'd like to keep this change. Here's my reasoning:

1. In general, we're moving away from returning updated MessageStore directly, in favor of returning operations.
2. I don't expect we'll ever need to introduce currentAsOf changes to reassignMessagesToRealizedThreads, as it's simply reassigning thread IDs.
3. I expect that anybody who would happen to change that would look downstream at all callsites of reassignMessagesToRealizedThreads to make sure their new changes are being picked up appropriately.
4. They would also need to introduce a new message store op, so they would probably just update the code here to be messageStore: processedMessageStore anyways.

Makes sense! I was more afraid about introducing a new field to the messageStore, but that would also require reviewing the reducers.