Direct messaging is the private side of Twitter. I worked on quite a few projects to help bring DMs up to date for a modern messaging platform, including a request inbox, typing indicators, read receipts, and settings management.
Twitter is all about real-time, yet DMs up until this point felt static and slow. There was no way to know that a conversation was active, so users would often go away only to have a message get sent to them rather quickly. There were quite a few explorations to refine how multiple users would appear and dissapear during the typing process, as well as what happens when a message is sent and how the message appears in the chat.