Design a real-time comment feed with optimistic updates

Design the front end of a comment section where other people's comments arrive live and the viewer can post their own. The focus is optimistic posting: a submitted comment shows up at the end of the list at once, before the server confirms it. Cover how new comments arrive (WebSocket, SSE or polling), how the pending comment is reconciled with the server's copy when it comes back, ordering and duplicates, what the user sees when the post fails and how they retry.