diff --git a/src/helpers/buffers/ordering_sender.rs b/src/helpers/buffers/ordering_sender.rs
index 5ecc21c7f..ee73b3cb4 100644
--- a/src/helpers/buffers/ordering_sender.rs
+++ b/src/helpers/buffers/ordering_sender.rs
@@ -78,12 +78,13 @@ impl State {
             M::Size::USIZE,
             self.spare.get()
         );
+        let open = self.accept_writes();
         let b = &mut self.buf[self.written..];
-        if M::Size::USIZE <= b.len() {
+        if open && M::Size::USIZE <= b.len() {
             self.written += M::Size::USIZE;
             m.serialize(GenericArray::from_mut_slice(&mut b[..M::Size::USIZE]));
 
-            if self.written + self.spare.get() >= self.buf.len() {
+            if !self.accept_writes() {
                 Self::wake(&mut self.stream_ready);
             }
             Poll::Ready(())
@@ -111,6 +112,15 @@ impl State {
         self.closed = true;
         Self::wake(&mut self.stream_ready);
     }
+
+    /// Returns `true` if more writes can be accepted by this sender.
+    /// If message size exceeds the remaining capacity, [`write`] may
+    /// still return `Poll::Pending` even if sender is open for writes.
+    ///
+    /// [`write`]: Self::write
+    fn accept_writes(&self) -> bool {
+        self.written + self.spare.get() < self.buf.len()
+    }
 }
 
 /// An saved waker for a given index.
@@ -259,6 +269,11 @@ impl Waiting {
 /// Data less than the `write_size` threshold only becomes available to
 /// the stream when the sender is closed (with [`close`]).
 ///
+/// Once `write_size` threshold has been reached, no subsequent writes
+/// are allowed, until stream is polled. `OrderingSender` guarantees equal
+/// size chunks will be sent to the stream when it is used to buffer
+/// same-sized messages.
+///
 /// The `spare` capacity determines the size of messages that can be sent;
 /// see [`send`] for details.
 ///
@@ -465,14 +480,14 @@ impl<B: Borrow<OrderingSender> + Unpin> Stream for OrderedStream<B> {
 
 #[cfg(all(test, any(unit_test, feature = "shuttle")))]
 mod test {
-    use std::{iter::zip, num::NonZeroUsize};
+    use std::{future::poll_fn, iter::zip, num::NonZeroUsize, pin::pin};
 
     use futures::{
         future::{join, join3, join_all},
         stream::StreamExt,
         FutureExt,
     };
-    use futures_util::future::try_join_all;
+    use futures_util::future::{poll_immediate, try_join_all};
     use generic_array::GenericArray;
     use rand::Rng;
     #[cfg(feature = "shuttle")]
@@ -677,4 +692,41 @@ mod test {
             .unwrap();
         });
     }
+
+    /// If sender is at capacity, but still have some bytes inside spare, we block the sends
+    /// until the stream is flushed. That ensures `OrderingSender` yields the equal-sized
+    /// chunks.
+    ///
+    /// This behavior is important for channels working in parallel `[parallel_join]` and wrapped
+    /// inside a windowed execution [`seq_join`]. Not enforcing this leads to some channels moving
+    /// forward faster and eventually getting outside of active work window. See [`issue`] for
+    /// more details.
+    ///
+    /// [`seq_join`]: crate::seq_join::SeqJoin::try_join
+    /// [`parallel_join`]: crate::seq_join::SeqJoin::parallel_join
+    /// [`issue`]: https://github.com/private-attribution/ipa/issues/843
+    #[test]
+    fn reader_blocks_writers() {
+        const SZ: usize = <<Fp32BitPrime as Serializable>::Size as Unsigned>::USIZE;
+        run(|| async {
+            const CAPACITY: usize = SZ + 1;
+            const SPARE: usize = 2 * SZ;
+            let sender =
+                OrderingSender::new(CAPACITY.try_into().unwrap(), SPARE.try_into().unwrap());
+
+            // enough bytes in the buffer to hold 2 items
+            for i in 0..2 {
+                sender
+                    .send(i, Fp32BitPrime::truncate_from(u128::try_from(i).unwrap()))
+                    .await;
+            }
+
+            // spare has enough capacity, but buffer is considered full.
+            let mut f = pin!(sender.send(2, Fp32BitPrime::truncate_from(2_u128)));
+            assert_eq!(None, poll_immediate(&mut f).await);
+
+            drop(poll_fn(|ctx| sender.take_next(ctx)).await);
+            assert_eq!(Some(()), poll_immediate(f).await);
+        });
+    }
 }