Laid-out scene for the CURRENT input, or null while the FIRST
layout of this input is in flight (or while input is null).
Same-input re-layouts return the prior scene until the new
geometry lands (stale-while-revalidate); a stale scene from a
PREVIOUS input is never returned.
Laid-out scene for the CURRENT input, or null while the FIRST layout of this input is in flight (or while input is null). Same-input re-layouts return the prior scene until the new geometry lands (stale-while-revalidate); a stale scene from a PREVIOUS input is never returned.