Seeing a floating "notification square icon" on my MIUI 12.5 phone - why?

Here I was, just living my life, a happy camper of a MIUI 12.5.1 phone (Mi 9 SE to be specific), and everything was just fine. Well, not exactly, but for the purposes of this question it was. Then, a couple of days ago, I noticed something very weird: The application launcher screens - my home screens - now had a square icon floating over them! It had the icon of a Signal user, and a smaller notification-like icon on its corner... I think it was the blue Signal app logo. When I would press the icon, A Signal (?) screen would come up with the message. I could only kill this window by long-pressing on it, then dragging it to an (x) icon which would appear at the bottom of the screen. But with a new message, it appeared again. For now I haven't gotten any new messages, so I don't know if I'll see it again or not. My questions: 1. Is this the Signal app's doing, or is it my MIUI? 2. What is this "feature" called? 3. How can I disable it permanently and for all apps?