Function in_window 

pub(in sync::v3) fn in_window(
    since: u64,
    next_batch: u64,
) -> impl Fn(u64) -> bool + Copy

The window a sync round reports a cursor advance in.

The lower bound is the round’s own token and the upper bound is the response cutoff captured before the room is assembled, so a cursor stamped after that cutoff falls outside it.