Answer
On mobile, swipe a delivered People message from left to right. On desktop web, hover and choose the eye-shaped Views control. Private AI and temporary chats do not have this control. Gestures are unavailable while a message is still being sent.
Use the reaction control to add a reaction, or long-press a message on mobile. On updated builds, long-press or double-tap the displayed reaction emojis to open Message Viewers, including in private AI and temporary chats. A single tap still opens reaction choices. Older builds keep the Message Reactions list.
Message Viewers shows confirmed viewers and stored reactors. Reactors appear first, then other confirmed viewers, with each group sorted by name. Hashchats appears once it reacts, but does not join Chat Participants or the header’s participant photos. Human read receipts stay unchanged.