Function hold_spare_screens 

pub fn hold_spare_screens(
    socket_path: &str,
    keep: usize,
    size: (i32, i32),
) -> Result<usize, String>

Hold every screen past the first keep at size. A session compositor opens the screens it was started with whether or not anything watches them, and one held at a real screen’s size stretches the session’s coordinate space onto a screen nobody sees. Returns how many were resized.