A CodeSandbox for demonstrating this issue. and it also happen in your demo home page .
When use in static document flow (not
position: fixed;), the message container will scroll down when fetched old messages in order to go back to previous position. However current implementation of this behavior  will also effect the outside scrollbar, and this behavior is awful in our scenario which will disturb user when seeing Live stream. I think the feasible solution is calculating the target scroll top and set it to the message container’s
scrollTop attribute instead of using