Keychat distinguishes between the ID and the receiving/sending addresses. The ID remains unchanged, while the receiving and sending addresses are updated continuously. The ID does not appear on the envelope; only the addresses are displayed.
Discussion
OK. So the ID is fixed, but the sending/receiving addresses are independently ratcheted (2 ratchets), and only the ratcheted addresses are exposed during transmit.
I'm so used to thinking of id's and addresses as the same because they are both identifiers. Here the identity is fixed and private while the addresses are ephemeral and public.