Function txns_progress_frontiers 

fn txns_progress_frontiers<'scope, K, V, T, D, P, C>(
    remap: StreamVec<'scope, T, DataRemapEntry<T>>,
    passthrough: StreamVec<'scope, T, P>,
    name: &str,
    data_id: ShardId,
    until: Antichain<T>,
    unique_id: u64,
) -> (StreamVec<'scope, T, P>, PressOnDropButton)
where
    K: Debug + Codec,
    V: Debug + Codec,
    T: Timestamp + Lattice + TotalOrder + StepForward + Codec64,
    D: Clone + 'static + Monoid + Codec64 + Send + Sync,
    P: Debug + Clone + 'static,
    C: TxnsCodec,

The block ordering inside the schedule closure is load-bearing: pending passthrough input is emitted at the pre-activation capability BEFORE any capability downgrade, which keeps the differential invariant send_time <= record_time and avoids dropping in-flight rows when the passthrough frontier crosses until in the same activation (SQL-299). Do not reorder.