The Stage reads the room for itself, with its own session, which is what makes it a display target rather than a mirror of the instructor's laptop. A window opened from the conductor does not inherit that session, so it asks for one here.

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Signing in here brings you back to this window with the classroom, the sitting and the layer in the address intact. Prefer a link by email? The staff door sends one, and it comes back there.

Driving this wall is done from the conductor, which sends this window a layer name and two ids over a channel that reaches only this machine. Nothing a child wrote travels it, because there is nothing on it that could hold any.