Peripheral window controls — play/pause, tab switches — placed outside the window body but still visually bound to it, rather than embedded in the content area. Move the window and the ornament follows, keeping the same relative position.
Two reasons to use one: it keeps the window's content area untouched, and the ornament can float a bit closer to the user than the window (a larger z value), so it always sits in the layer your hand reaches first.
The demo floats a small pill-shaped toolbar just below a glass window, slightly closer to the viewer — it sways together with the window but on a subtly different phase, reading as the same group at a different depth.
When to use
Use it for global controls unrelated to the window content — close, playback controls, tabs. Keep content-specific buttons inside the window.