Why is my terminal letterboxed, with a badge naming another device?

tmux sizes a shared window to its smallest attached client. So when another device — a laptop, say — is attached to the same session at a different size, dotMux adopts that size and letterboxes the terminal to match, with a badge that names the other client.

Type anything, or tap the badge, to reclaim your full size. It isn’t a bug; it’s tmux keeping every client’s view of the window consistent.