Starting from the question rather than from the components. A thread is a scroll region that follows its own tail, so the follow belongs to the region, not to the page that happens to render it.

The pieces already existed, so the template is mostly arrangement:

Conversation owns the follow: it pins to the newest turn while you are at the end, and lets go the moment you scroll away.
Message owns a turn's anatomy — the body, and the actions that appear once you are done reading.
PromptInput owns the draft, and ThinkingBlock owns the reasoning behind an answer.

Swap the transport for a real one and nothing above it moves.

Chat app

Assistant

Canvas app