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.
Why the layout of the CURRENT input failed, or null.
Without this a host cannot tell "still laying out" from "permanently failed": both read as
structural === null, so a rejected layout spun a loading state forever and reported nothing but a global unhandledrejection. Render the message instead. An error from a PREVIOUS input is never returned, and a later successful layout of the same input clears it.