Reddit's 2016 April Fools' event that used an IRC-like chat window was Robin.
For April Fools' Day 2016, Reddit launched Robin as a social experiment. After clicking the Robin button, a participant was placed in an IRC-like chat with another user and given a limited choice about what should happen next. The event turned a simple interface into a test of cooperation, coordination, and group decision-making.
Participants could choose “Grow,” which moved them into a larger group; “Stay,” which closed the group chat and created a subreddit with that group as moderators; or “Leave,” which ended the interaction. The structure made every small decision affect the size and future of the group.
Robin is sometimes confused with Reddit's other April Fools' experiments. The Button was the 2015 event, Place was the collaborative pixel canvas from 2017, and Sequence was introduced in 2019. Robin specifically belongs to 2016 and is the one associated with the chat-window experiment.