Rest the pointer on the notch and it opens onto your shelf, your music and your widgets. Move away and it tucks back in.

Built into Ledge for macOS.
Rest the pointer and it opens, or click it if you turn hover off. It closes a moment after you leave.
The pill under the open notch moves between the shelf, the player and your widgets.
Right-click to hide it. It also steps aside for apps you choose and while an app fills the screen.