From d7ffb95374c822629f6faf886e3a06302f3db01a Mon Sep 17 00:00:00 2001
From: "Dylan M. Taylor" <dylan@dylanmtaylor.com>
Date: Tue, 3 Nov 2026 14:42:25 +0000
Subject: [PATCH] fix(export): encode WebM audio packets in place instead of
 draining each from the front

---
 crates/export/src/webm.rs     | 8 +++++---
 crates/export/src/webm_av1.rs | 8 +++++---
 2 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/crates/export/src/webm.rs b/crates/export/src/webm.rs
index a12bd4e1a..976eaf6e0 100644
--- a/crates/export/src/webm.rs
+++ b/crates/export/src/webm.rs
@@ -244,12 +244,14 @@ pub(crate) async fn webm(job: &Cx<'_>, comp: &Comp, w: u32, h: u32, st: &mut Sta
             let pad = (fs - pcm_left.len() % fs) % fs + fs;
             pcm_left.extend(std::iter::repeat_n(0.0, pad));
         }
-        while pcm_left.len() >= fs {
-            let pkt = enc.encode_float(&pcm_left[..fs]);
-            pcm_left.drain(..fs);
+        // Packets are read in place and the encoded samples dropped once: draining each packet
+        // from the front would shift the rest of the batch's audio every 20 ms.
+        for packet in pcm_left.chunks_exact(fs) {
+            let pkt = enc.encode_float(packet);
             frames.push(Frame { ms: packets as i64 * 20, track: 2, key: true, prev_ms: None, data: pkt, alpha: None });
             packets += 1;
         }
+        pcm_left.drain(..pcm_left.len() / fs * fs);
     };
 
     let batch = batch_size();
diff --git a/crates/export/src/webm_av1.rs b/crates/export/src/webm_av1.rs
index ca06a3e51..6fc26ac85 100644
--- a/crates/export/src/webm_av1.rs
+++ b/crates/export/src/webm_av1.rs
@@ -124,12 +124,14 @@ pub(crate) async fn webm_av1(job: &Cx<'_>, comp: &Comp, w: u32, h: u32, st: &mut
             let pad = (fs - pcm_left.len() % fs) % fs + fs;
             pcm_left.extend(std::iter::repeat_n(0.0, pad));
         }
-        while pcm_left.len() >= fs {
-            let pkt = enc.encode_float(&pcm_left[..fs]);
-            pcm_left.drain(..fs);
+        // Packets are read in place and the encoded samples dropped once: draining each packet
+        // from the front would shift the rest of the batch's audio every 20 ms.
+        for packet in pcm_left.chunks_exact(fs) {
+            let pkt = enc.encode_float(packet);
             blocks.push(Block { ms: packets as i64 * 20, track: 2, key: true, data: pkt });
             packets += 1;
         }
+        pcm_left.drain(..pcm_left.len() / fs * fs);
     };
 
     let batch = batch_size();