Function cover 

pub fn cover<R, S>(
    open: S,
    build: impl Fn(&R) -> AnyPiece + 'static,
) -> Cover<S, R>
where
    R: Route,
    S: SignalRw<Option<R>>,

A fullscreen cover over open: Some(r) presents build(&r), None dismisses (docs/cover.md). Registers a string-route adapter, so navigate("<key>") opens it and nav_back() closes it, and current_route() reports the presented key.