Function windows_on_output 

pub fn windows_on_output(
    space: &Space<Window>,
    display_id: u32,
) -> impl Iterator<Item = &Window>

Every window placed on display_id and actually composited there. Keyed off the window’s own output tag rather than the Space’s output map, which only catches up on the next Space::refresh; a parked window carries the tag but is mapped clear of every output.