Answer
When your answer fills the screen, its header stays at the top so you can read from the start while the rest writes below. Whenever your answer is still writing and you are away from the newest messages, the round Latest button at the bottom shows a ring around its arrow.
Scroll back down to within a few lines of the newest words, or tap Latest, and Hashchats keeps you at the bottom: new lines slide into view until the answer finishes, and you stay at the bottom afterwards. Scroll up again to stop following and keep your place.
A short answer at the bottom of the chat stays still when it finishes: room for its reactions, Copy and other actions is kept while it writes. A link preview that arrives afterwards is added below the text instead of pushing it up; if it lands below the screen, the Latest button takes you to it.
In illustrated articles, a picture that opens above the part you are reading does not push your lines down. With reduced motion, following jumps line by line instead of sliding. This only applies to the answer you asked for; other people's answers follow the chat as usual.