this line should be conditional depending on canUseDatabaseOnWeb introduced in D9948

It might be worth considering a scenario where a staff user hasn't accepted updated policies when they load the web app. In that case, would threads.length === 0 for that staff user?

(Not sure if this is a concern or not, just figured it's relevant to @tomek's question)

is it possible for a staff user to have threads.length === 0 in the DB?

Yes, this is the case pointed out by @ashoat.

Do we have a way of determining if a user is a staff other than checking the length?

It's not for determining if a user is a staff, it's for determining whether threads are in a table or not

1. If threads are in DB, we read it and populate the store
2. If threads are not in DB (length 0) we could have 4 scenarios:

a. this is a non-staff user, we don't want to treat an empty array as a source of truth and override the state with empty threads, so we return a null
b. this is a staff user, so it means threads are not migrated yet, we return null to fetch it from the keyserver (D9959)
c. user has not acknowledged policies, so returning null is also okay, but doesn't change anything
d. user doesn't have any threads, and if it's possible or not, in the worst case it will ask the keyserver for threads because logic from D9959 will think it's case b, and keyserver will return an empty object again, but this is a really corner case, as right now each user has at least one thread.

so I think it's safe to keep this condition with length > 0

It might be worth considering a scenario where a staff user hasn't accepted updated policies when they load the web app. In that case, would threads.length === 0 for that staff user?

(Not sure if this is a concern or not, just figured it's relevant to @tomek's question)

That case applies to only changed policies when the user is logged out, when the user logs in thread length is 0 anyway, so this change has no influence on policies logic.

In this diff, I just discovered I missed this change while creating a diff and made a note to myself and the reviewers. Adding this right now