Conversations involving multiple participants, allowing collaboration and information sharing in Teams
That's what I thought the issue was when I was initially investigating it a few months ago, but it only makes up half of the reported issues from what I can see, and also what happens to me.
If I move my mouse to that side of the screen before typing then it'll just go straight to the person in the list who's name appears with the same letter as I'm typing, but when I move my mouse to the other side of the screen, I can get a few replies in before it jumps to a different chat. Usually a previous chat? Same behaviour with the message reacts.