Function defer_arriving 

pub fn defer_arriving(
    app_id: &'static str,
    title: &'static str,
    arrival_shape: SkeletonShape,
    load: fn() -> Pin<Box<dyn Future<Output = Option<&'static ApplicationDefinition>> + Send>>,
) -> bool

defer with the shape the app’s window shows while its code is on its way: what the app declares for it in its manifest.