Answer
This happens in private AI chats and People chats, in desktop and phone browsers and in the updated iPhone and Android apps, on the device or tab you sent the request from, while you stay in that chat. Once words start appearing, a small spinner in the answer's header shows until the text and any previews, places, images or narration audio finish.
If you are at the latest messages, the answer grows in place and keeps its header at the top once it fills the screen, including answers that arrive all at once. A short answer stays at the bottom. Scrolling yourself stops that. Web search answers start writing after research finishes. In illustrated articles, each picture opens in place once it has loaded. Spoken answers write live too, and their audio player follows when the narration is ready. Files, cards, quizzes and polls appear when they are finished. Image, video and music results keep their usual loader.
Leaving the chat or sending another message returns an unfinished answer to the usual loader. Only your newest request writes live. With reduced motion, text appears a line at a time. Writing live does not change your AI usage.