Keep a floating picture-in-picture window (browser PiP, mpv, a webcam, etc.) visible on every workspace you switch to — the "pin on all workspaces" behaviour other compositors have, for niri.
niri has no built-in sticky-window mode yet (niri#932). This is a ~30-line daemon that reaches the same result: it listens to niri's JSON event stream and moves the PiP window to whatever workspace you focus. No polling, no CPU while you sit still.