I think all that is the responsibility of the viewer. When you view a single message on Mastodon, then Mastodon is responsible for fetching the rest of the thre

I think all that is the responsibility of the viewer. When you view a single message on Mastodon, then Mastodon is responsible for fetching the rest of the thread for your display. In practice, it looks like Mastodon also stores / caches local copies of messages and profiles, pretty much as they are received. (Including thumbnails!)