A replied thread is updated with the reply timestamp and shown at the bottom of the message list.
You can paste URLs in chat messages; links are generated automatically. (Tapping an auto-generated link opens the page in an external browser.)
Files attached directly to chat messages are available for 30 days. After 30 days attachments may be removed, and you may see a 403 or 404 when attempting to access them.
When you access a chat room for the first time, the last 30 days of messages are retrieved. Press View previous messages to load older messages.