A thread should only be marked as unread if:

1. latestReadMessage is not set. In general, we only bump last_message_for_unread_check (formerly last_message) and don't bump last_read_message, but there are two exceptions: the member's self-authored messages, and messages in threads are currently in-focus by the member (see lines 557-558)
2. The new check, latestMessageForUnreadCheck. Previously we would always bump last_message, but now for last_message_for_unread_check, in certain cases we will not bump it (defined in later diffs inside message specs).

You might ask why we needed a new mechanism here. Could we have handled not setting certain threads to unread by simply updating last_message? The reason we couldn't do this was because in the case of a thread that is currently unread, we would end up making it unread if we bumped last_read_message alongside last_message. This behavior is okay for the member's self-authored messages (sending a message always sets that thread to read for the author) and threads that are currently in-focus (we always want to set them to be read), but it's not okay for the new feature.

We could consider replacing the old mechanism with the new one, but I decided to avoid that to keep scope limited.

Correction on this part:

Could we have handled not setting certain threads to unread by simply updating last_message? The reason we couldn't do this was because in the case of a thread that is currently unread, we would end up making it unread if we bumped last_read_message alongside last_message.

Should have been:

Could we have handled not setting certain threads to unread by simply updating last_read_message? The reason we couldn't do this was because in the case of a thread that is currently unread, we would end up making it read if we bumped last_read_message alongside last_message.