What a surface template receives: its key, as let-key; whether it is on screen, as let-visible="Visible"; and its placement, as let-placement="Placement".
let-key
let-visible="Visible"
let-placement="Placement"
Where the surface is placed.
Whether the surface is on screen now: its placement shows it and the stage itself is on screen.
What a surface template receives: its key, as
let-key; whether it is on screen, aslet-visible="Visible"; and its placement, aslet-placement="Placement".