Constant FLUSHER_EOF_GRACE 

pub(super) const FLUSHER_EOF_GRACE: Duration;

How long to wait for the STRONG completion signal — the flusher task ending, which means the read loop saw true EOF and every byte the child produced is committed — before falling back to the bounded flush barrier.

Short on purpose. Where EOF works (Linux, macOS) it arrives within milliseconds of the child being reaped, so this grace is ample and the pre-barrier guarantee is kept intact. Where it never arrives at all (Windows/ConPTY — see FLUSHER_DRAIN_TIMEOUT) no amount of waiting helps, so anything longer is pure dead UI time.