Surfaces the user hid, so the UI can offer them back.
Participants not already on screen (on the stage or in a PiP), for a filmstrip or grid; the user first when shown.
Picture-in-picture surfaces, newest first.
The screen being shared and the current speaker, for a split view; null when nobody is sharing.
What fills the stage: a surface placed there, else the spotlight participant; null for an empty session.
Surfaces shown as tabs, in the order they appeared.
Where everything goes.