Over the years, Twitter has undergone many changes. Some recent ones that come to mind include the threading and the 280 character limit. The latest changes the company is testing include a new thread interface, as well as a presence feature that will be shown when people are online.
The most notable test of the two is the new user interface for tweet threads. Each tweet now gets its own bubble, and the responses of the original tweeter receive a fill of color. While this makes it clearer who responds to what, it seems that many, including myself, are not fans of the overall appearance. Some even complained that Twitter looks too much like Facebook.
You may also notice some small green dots on top of some avatars in the first screenshot, which are status indicators that show if someone is currently on Twitter or not. This is also something that Facebook implemented first. If this feature is made official, we can only hope that it can be deactivated, as does Instagram with its state indicator points for DM.