Function offer 

pub fn offer<S>(
    slots: &mut WindowSlots,
    windows: &[OpenWindow],
    shared: &[SharedWindow],
    offered: &mut HashSet<u64, S>,
)
where
    S: BuildHasher + Default,

Put each of windows’ share item in its menu, and take it out of the menus of the windows in offered (the ones it stood in) that are no longer among them. offered becomes the windows it stands in now.