diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 22ebe40685..4a7e2343cc 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -271,10 +271,10 @@ jobs:
 
   test:
     name: Test
-    needs: [changes, producer-source-tests]
-    # Keep the existing required `Test` context authoritative for producer
-    # failures too. The dedicated producer matrix remains parallel and legible,
-    # while this job fails closed if either lane fails or is cancelled.
+    needs: [changes, producer-source-tests, audio-pad-ffmpeg7]
+    # Keep the existing required `Test` context authoritative for the producer
+    # and pinned-FFmpeg lanes too. Those stay parallel and legible on their
+    # own, while this job fails closed if any of them fails or is cancelled.
     if: always() && needs.changes.outputs.code == 'true'
     runs-on: ubuntu-latest
     timeout-minutes: 10
@@ -284,6 +284,11 @@ jobs:
         run: |
           echo "::error::Producer unit/integration tests did not succeed."
           exit 1
+      - name: Require the pinned-FFmpeg audio lane
+        if: needs.audio-pad-ffmpeg7.result != 'success'
+        run: |
+          echo "::error::Audio pad chain tests on pinned FFmpeg 7.x did not succeed."
+          exit 1
       - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
         with:
           lfs: true
@@ -332,6 +337,75 @@ jobs:
       - run: bun run --filter @hyperframes/engine build
       - run: bun run producer:test:${{ matrix.lane }}
 
+  # The audio pad/trim chain regressed on the FFmpeg 7.x line only: `atrim`
+  # reading an indefinite `apad`'s timestamps drops the last-delayed branch
+  # from a mix of four or more. No other lane can catch it. `Test` has no
+  # FFmpeg at all (its gated audio tests report as skipped), and every FFmpeg
+  # the other lanes do get is unaffected: `apt-get install ffmpeg` on the
+  # runner image is the 6.x line, and the Windows lane pins an 8.x master
+  # build. Both produce correct output from the broken chain, so the
+  # regression test passes on them whether or not the fix is present.
+  #
+  # So this lane exists to pin one build that actually fails without the fix.
+  # Verified on linux/amd64 against the pinned 7.1.3 below: with the fix the
+  # test passes, and with `buildPadToDurationFilter` reverted to the bare
+  # `apad,atrim=0:<total>` it fails on the `t=0` silence assertion.
+  #
+  # Same pinning rule as .github/actions/install-ffmpeg-windows: the asset
+  # filename embeds the git hash, so the tag and the filename must be bumped
+  # together. Do not float this to `latest` — `latest` carries only 8.x and
+  # 9.x, neither of which reproduces the bug.
+  audio-pad-ffmpeg7:
+    name: "Audio: pad chain on FFmpeg 7.x"
+    needs: changes
+    if: needs.changes.outputs.code == 'true'
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    env:
+      FFMPEG_7_URL: https://github.com/BtbN/FFmpeg-Builds/releases/download/autobuild-2026-03-31-13-11/ffmpeg-n7.1.3-43-g5a1f107b4c-linux64-gpl-7.1.tar.xz
+    steps:
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
+        with:
+          lfs: true
+      - uses: oven-sh/setup-bun@0c5077e51419868618aeaa5fe8019c62421857d6 # v2
+      - uses: actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020 # v4
+        with:
+          node-version: 22
+      - name: Install pinned FFmpeg 7.x
+        run: |
+          set -euo pipefail
+          dir="$RUNNER_TEMP/ffmpeg7"
+          mkdir -p "$dir"
+          for attempt in 1 2 3 4 5; do
+            if curl -fsSL --retry 3 -o "$RUNNER_TEMP/ffmpeg7.tar.xz" "$FFMPEG_7_URL"; then
+              break
+            fi
+            echo "download attempt $attempt failed"
+            if [ "$attempt" = 5 ]; then exit 1; fi
+            sleep $((15 * attempt))
+          done
+          tar -xf "$RUNNER_TEMP/ffmpeg7.tar.xz" -C "$dir" --strip-components=2 --wildcards '*/bin/ffmpeg' '*/bin/ffprobe'
+          chmod +x "$dir/ffmpeg" "$dir/ffprobe"
+          "$dir/ffmpeg" -version | head -n 1
+          # The engine resolves both binaries through these two variables, so
+          # the skip gate and the code under test agree on which build ran.
+          echo "HYPERFRAMES_FFMPEG_PATH=$dir/ffmpeg" >> "$GITHUB_ENV"
+          echo "HYPERFRAMES_FFPROBE_PATH=$dir/ffprobe" >> "$GITHUB_ENV"
+      - uses: ./.github/actions/prepare-ffmpeg-bin
+      - run: bash scripts/ci/install-workspace-dependencies.sh
+      - run: bun run --filter '@hyperframes/{parsers,lint,studio-server}' build
+      - run: bun run --cwd packages/core build
+      # Fail loudly rather than skipping: a silent skip here is the exact
+      # failure this lane was added to remove.
+      - name: Assert the pinned FFmpeg is the one under test
+        run: |
+          set -euo pipefail
+          test -x "$HYPERFRAMES_FFMPEG_PATH"
+          "$HYPERFRAMES_FFMPEG_PATH" -version | head -n 1 | grep -q 'version n7\.'
+      - run: >-
+          npx vitest run --root packages/engine
+          src/services/audioMixer.padTimestamps.integration.test.ts
+
   # Tests under skills/**/*.test.mjs are bare `node --test` files with only
   # `node:` built-in imports. They aren't part of any workspace package, and
   # the main `Test` job's `code` path filter excludes `skills/**`, so without
diff --git a/packages/engine/src/index.ts b/packages/engine/src/index.ts
index b8f8177f3b..f08fcfa70c 100644
--- a/packages/engine/src/index.ts
+++ b/packages/engine/src/index.ts
@@ -218,6 +218,7 @@ export {
   parseAudioElements,
   processCompositionAudio,
 } from "./services/audioMixer.js";
+export { buildPadToDurationFilter } from "./services/audioPadFilter.js";
 export { cloneCaptureWarning, cloneCaptureWarnings } from "./services/captureWarning.js";
 export type {
   AudioElement,
diff --git a/packages/engine/src/services/audioMixer.padTimestamps.integration.test.ts b/packages/engine/src/services/audioMixer.padTimestamps.integration.test.ts
new file mode 100644
index 0000000000..15defa424f
--- /dev/null
+++ b/packages/engine/src/services/audioMixer.padTimestamps.integration.test.ts
@@ -0,0 +1,149 @@
+import { execFileSync, spawnSync } from "node:child_process";
+import { mkdtempSync, rmSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { afterEach, describe, expect, it } from "vitest";
+import { getFfmpegBinary, getFfprobeBinary } from "../utils/ffmpegBinaries.js";
+import { processCompositionAudio } from "./audioMixer.js";
+import type { AudioElement } from "./audioMixer.types.js";
+
+/**
+ * Real-FFmpeg regression test for the pad/trim branch template.
+ *
+ * `apad,atrim=0:<total>` — an indefinite pad bounded by a downstream trim —
+ * behaves on FFmpeg 4.2.7 and misbehaves on 7.0.2 and newer: audio leaks to
+ * `t=0` from three mixed branches onward, and from four branches onward the
+ * branch with the largest `adelay` disappears from the mix. Nothing errors;
+ * the render succeeds with wrong audio, which is why only an output
+ * measurement catches it.
+ *
+ * Three or fewer clips is not enough — the dropped-branch half of the bug does
+ * not appear until four. This mixes five.
+ */
+
+const dirs: string[] = [];
+afterEach(() => dirs.splice(0).forEach((dir) => rmSync(dir, { recursive: true, force: true })));
+
+// Resolve the same binary `processCompositionAudio` will, so the gate and the
+// code under test cannot disagree under `HYPERFRAMES_FFMPEG_PATH`, which is
+// exactly how you point this test at a specific FFmpeg to reproduce the bug.
+const FFMPEG = getFfmpegBinary();
+
+const hasFfmpeg = (() => {
+  try {
+    execFileSync(FFMPEG, ["-version"], { stdio: "ignore" });
+    execFileSync(getFfprobeBinary(), ["-version"], { stdio: "ignore" });
+    return true;
+  } catch {
+    return false;
+  }
+})();
+
+const TOTAL_SECONDS = 43.3;
+
+/** start (s), source duration (s), tone frequency (Hz) */
+const CLIPS = [
+  { start: 0.3, duration: 7.01, frequency: 400 },
+  { start: 19.6, duration: 0.4, frequency: 1000 },
+  { start: 20.6, duration: 0.4, frequency: 1200 },
+  { start: 30.0, duration: 0.4, frequency: 1500 },
+  { start: 40.0, duration: 0.4, frequency: 1800 },
+] as const;
+
+/**
+ * Mean RMS in dB over a 250ms window starting at `atSeconds`. Digital silence
+ * reports `-inf`, which is the signal for "this branch is not in the mix".
+ */
+function rmsDb(path: string, atSeconds: number): number {
+  const probe = spawnSync(
+    FFMPEG,
+    [
+      "-hide_banner",
+      "-v",
+      "info",
+      "-ss",
+      atSeconds.toFixed(3),
+      "-t",
+      "0.25",
+      "-i",
+      path,
+      "-af",
+      "astats=metadata=1",
+      "-f",
+      "null",
+      "-",
+    ],
+    { encoding: "utf8" },
+  );
+  const stderr = probe.stderr ?? "";
+  const overall = stderr.slice(stderr.lastIndexOf("Overall"));
+  const match = /RMS level dB:\s*(\S+)/.exec(overall);
+  if (!match) throw new Error(`astats reported no RMS level for ${path} at ${atSeconds}s`);
+  const raw = match[1]!;
+  return raw.endsWith("inf")
+    ? raw.startsWith("-")
+      ? -Infinity
+      : Infinity
+    : Number.parseFloat(raw);
+}
+
+describe.skipIf(!hasFfmpeg)("mixed audio branch padding", () => {
+  it("keeps t=0 silent and lands all five staggered clips at their offsets", async () => {
+    const baseDir = mkdtempSync(join(tmpdir(), "hf-padts-base-"));
+    const workDir = mkdtempSync(join(tmpdir(), "hf-padts-work-"));
+    dirs.push(baseDir, workDir);
+
+    const elements: AudioElement[] = CLIPS.map((clip, i) => {
+      const src = `clip-${i}.wav`;
+      execFileSync(FFMPEG, [
+        "-hide_banner",
+        "-loglevel",
+        "error",
+        "-f",
+        "lavfi",
+        "-i",
+        `sine=frequency=${clip.frequency}:duration=${clip.duration}`,
+        "-ar",
+        "44100",
+        "-ac",
+        "2",
+        "-y",
+        join(baseDir, src),
+      ]);
+      return {
+        id: `clip-${i}`,
+        src,
+        start: clip.start,
+        end: clip.start + clip.duration,
+        mediaStart: 0,
+        layer: 0,
+        volume: 1,
+        type: "audio",
+      };
+    });
+
+    const outputPath = join(baseDir, "out.m4a");
+    const result = await processCompositionAudio(
+      elements,
+      baseDir,
+      workDir,
+      outputPath,
+      TOTAL_SECONDS,
+    );
+
+    expect(result.error).toBeUndefined();
+    expect(result.success).toBe(true);
+    expect(result.tracksProcessed).toBe(CLIPS.length);
+
+    // The earliest clip starts at 0.3s, so the head of the mix is silence.
+    // The bug puts every clip's audio here at once, ~-29 dB.
+    expect(rmsDb(outputPath, 0)).toBeLessThan(-60);
+
+    // Every clip is audible at its own offset. `-Infinity` here means the
+    // branch is missing from the mix entirely, which is the second half of
+    // the bug and only shows from four branches onward.
+    for (const clip of CLIPS) {
+      expect(rmsDb(outputPath, clip.start + 0.02)).toBeGreaterThan(-45);
+    }
+  }, 120_000);
+});
diff --git a/packages/engine/src/services/audioMixer.test.ts b/packages/engine/src/services/audioMixer.test.ts
index 341c9130da..b41d37d471 100644
--- a/packages/engine/src/services/audioMixer.test.ts
+++ b/packages/engine/src/services/audioMixer.test.ts
@@ -296,7 +296,7 @@ describe("processCompositionAudio", () => {
 
     expect(filter).toContain("volume=0");
     expect(filter).toContain("[mixed]volume=1[out]");
-    expect(filter).toContain("apad,atrim=0:2");
+    expect(filter).toContain("apad,asetpts=N/SR/TB,atrim=0:2");
     expect(filter).not.toContain("whole_dur");
     expect(filter).not.toContain("normalize=");
     expect(filter).not.toContain("weights=");
@@ -464,7 +464,7 @@ describe("processCompositionAudio", () => {
     // 2 s clip + the 1.9 s tail 0.6 + size * 2.6 generates.
     expect(filter).toContain("atrim=0:3.9,");
     // And still cut at the composition's end, so a tail cannot extend the video.
-    expect(filter).toContain("apad,atrim=0:8");
+    expect(filter).toContain("apad,asetpts=N/SR/TB,atrim=0:8");
   });
 
   it("hands the volume envelope to the FX pass instead of ducking the file after it", async () => {
@@ -1076,6 +1076,9 @@ describe("processCompositionAudio", () => {
     // indefinite `apad` to cap the padded stream at composition duration.
     expect((filter?.match(/atrim=/g) ?? []).length).toBe(trackCount * 2);
     expect((filter?.match(/apad,/g) ?? []).length).toBe(trackCount);
+    // The timestamp rebuild between the two is what keeps that cap correct on
+    // FFmpeg 7+; without it the last-delayed track is dropped from the mix.
+    expect((filter?.match(/apad,asetpts=N\/SR\/TB,atrim=/g) ?? []).length).toBe(trackCount);
   });
 
   it("retries with the current file-valued filter option when a nightly removes the legacy alias", async () => {
diff --git a/packages/engine/src/services/audioMixer.ts b/packages/engine/src/services/audioMixer.ts
index 2d856d1d7e..2153c02da2 100644
--- a/packages/engine/src/services/audioMixer.ts
+++ b/packages/engine/src/services/audioMixer.ts
@@ -31,6 +31,7 @@ import type {
   MixResult,
 } from "./audioMixer.types.js";
 import { applyVolumeEnvelopeToWav } from "./audioVolumeEnvelope.js";
+import { buildPadToDurationFilter } from "./audioPadFilter.js";
 import { HF_AUDIO_FX_ATTR, parseAudioFxChain } from "@hyperframes/core/audio-fx";
 import {
   HF_AUDIO_AUTOMATION_ATTR,
@@ -710,7 +711,7 @@ async function mixAudioTracks(
       const trimDuration = track.end - track.start + (track.tailSeconds ?? 0);
       const volumeFilter = buildVolumeExpression(track, ignoreAutomation);
       filterParts.push(
-        `[${i}:a]atrim=0:${formatFilterNumber(trimDuration)},${volumeFilter},adelay=${delayMs}|${delayMs},apad,atrim=0:${formatFilterNumber(totalDuration)}[a${i}]`,
+        `[${i}:a]atrim=0:${formatFilterNumber(trimDuration)},${volumeFilter},adelay=${delayMs}|${delayMs},${buildPadToDurationFilter(formatFilterNumber(totalDuration))}[a${i}]`,
       );
     });
 
diff --git a/packages/engine/src/services/audioPadFilter.test.ts b/packages/engine/src/services/audioPadFilter.test.ts
new file mode 100644
index 0000000000..9251e77b22
--- /dev/null
+++ b/packages/engine/src/services/audioPadFilter.test.ts
@@ -0,0 +1,27 @@
+import { describe, expect, it } from "vitest";
+import { buildPadToDurationFilter } from "./audioPadFilter.js";
+
+describe("buildPadToDurationFilter", () => {
+  it("pads indefinitely, rebuilds timestamps, then bounds at the target", () => {
+    expect(buildPadToDurationFilter("43.3")).toBe("apad,asetpts=N/SR/TB,atrim=0:43.3");
+  });
+
+  it("keeps off `whole_dur`, which the bundled Windows FFmpeg builds reject", () => {
+    expect(buildPadToDurationFilter("5.000000")).not.toContain("whole_dur");
+  });
+
+  it("uses the caller's number formatting verbatim", () => {
+    // Each call site formats seconds differently (fixed-6 in the producer,
+    // trimmed in the mixer). The helper must not normalize either away.
+    expect(buildPadToDurationFilter("5.000000")).toContain("atrim=0:5.000000");
+    expect(buildPadToDurationFilter("5")).toContain("atrim=0:5");
+  });
+
+  it("places asetpts between apad and atrim, which is the whole fix", () => {
+    // `atrim` reading an indefinite `apad`'s timestamps directly is what drops
+    // and misplaces mixed clips on FFmpeg 7+. Order is the behaviour here, so
+    // pin it rather than only pinning membership.
+    const chain = buildPadToDurationFilter("8").split(",");
+    expect(chain).toEqual(["apad", "asetpts=N/SR/TB", "atrim=0:8"]);
+  });
+});
diff --git a/packages/engine/src/services/audioPadFilter.ts b/packages/engine/src/services/audioPadFilter.ts
new file mode 100644
index 0000000000..57617505f6
--- /dev/null
+++ b/packages/engine/src/services/audioPadFilter.ts
@@ -0,0 +1,44 @@
+/**
+ * Pad-to-duration filter chain, shared by every audio path that has to hold a
+ * stream to the composition's length.
+ *
+ * Two live call sites build this chain (the engine mixer and the producer's
+ * pad/trim step), plus the producer's audio extractor, which is currently
+ * unreferenced. Before this module they each rebuilt the string by hand, so a
+ * fix in one did not reach the others.
+ *
+ * ## Why the chain is three filters and not one
+ *
+ * `apad` with no duration pads indefinitely; the trailing `atrim` is what
+ * bounds it. That pairing is deliberate: `apad=whole_dur=<seconds>` says the
+ * same thing in one filter, but the bundled Windows FFmpeg builds reject the
+ * option outright (`Error applying option 'whole_dur': Option not found`), and
+ * it does not bound a branch that is already *longer* than the target — an FX
+ * tail that overruns the composition survives `whole_dur` and is cut by
+ * `atrim`.
+ *
+ * On the FFmpeg 7.x line, however, `atrim` reading `apad`'s output timestamps
+ * directly is what broke the mix: audio leaked to `t=0` from three mixed
+ * branches onward, and from four branches onward the branch with the largest
+ * `adelay` vanished from the output entirely. Nothing errored.
+ *
+ * Measured on linux/amd64, the affected window is 7.x only: 4.2.7, 6.0.1, git
+ * master from 2026-05 and 8.1.1 all produce correct output from the bare
+ * `apad,atrim` pairing, and produce byte-identical output with the `asetpts`
+ * in place. So this chain is a no-op everywhere except the versions it fixes,
+ * and it stays because the next release to regress here cannot be predicted.
+ *
+ * `asetpts=N/SR/TB` between them rebuilds each frame's timestamp from the
+ * running sample count, so `atrim` sees a monotonic sample-accurate timeline
+ * instead of whatever the release propagates through an indefinite `apad`.
+ * Measured against FFmpeg 7.0.2, the output is then sample-for-sample what
+ * `apad=whole_dur` produces — without giving up the `atrim` bound or the
+ * Windows builds.
+ *
+ * @param seconds Target duration, already formatted for a filter string by the
+ *   caller. Each call site has its own number formatting and this helper must
+ *   not silently change it.
+ */
+export function buildPadToDurationFilter(seconds: string): string {
+  return `apad,asetpts=N/SR/TB,atrim=0:${seconds}`;
+}
diff --git a/packages/producer/src/services/audioExtractor.ts b/packages/producer/src/services/audioExtractor.ts
index ccddf32055..f4d3d390e3 100644
--- a/packages/producer/src/services/audioExtractor.ts
+++ b/packages/producer/src/services/audioExtractor.ts
@@ -9,7 +9,7 @@
 import { spawn } from "node:child_process";
 import { existsSync, mkdirSync, rmSync, readFileSync } from "node:fs";
 import { join, dirname } from "node:path";
-import { getFfmpegBinary, trackChildProcess } from "@hyperframes/engine";
+import { buildPadToDurationFilter, getFfmpegBinary, trackChildProcess } from "@hyperframes/engine";
 
 export interface AudioElement {
   id: string;
@@ -212,7 +212,7 @@ async function mixTracks(
     const trimDuration = track.duration > 0 ? track.duration : totalDuration;
 
     filterParts.push(
-      `[${i}:a]atrim=0:${trimDuration},volume=${track.volume},adelay=${delayMs}|${delayMs},apad,atrim=0:${totalDuration}[a${i}]`,
+      `[${i}:a]atrim=0:${trimDuration},volume=${track.volume},adelay=${delayMs}|${delayMs},${buildPadToDurationFilter(String(totalDuration))}[a${i}]`,
     );
   });
 
diff --git a/packages/producer/src/services/render/audioPadTrim.test.ts b/packages/producer/src/services/render/audioPadTrim.test.ts
index 6ea808c94b..a567d930ea 100644
--- a/packages/producer/src/services/render/audioPadTrim.test.ts
+++ b/packages/producer/src/services/render/audioPadTrim.test.ts
@@ -30,7 +30,7 @@ describe("buildPadTrimAudioArgs", () => {
     expect(plan.steps).toHaveLength(1);
     const args = plan.steps[0]!.args;
     expect(args[args.indexOf("-i") + 1]).toBe("/tmp/in.aac");
-    expect(args[args.indexOf("-af") + 1]).toBe("apad,atrim=0:5.000000");
+    expect(args[args.indexOf("-af") + 1]).toBe("apad,asetpts=N/SR/TB,atrim=0:5.000000");
     expect(args.join(" ")).not.toContain("whole_dur");
     expect(args[args.indexOf("-t") + 1]).toBe("5.000000");
     expect(args[args.indexOf("-c:a") + 1]).toBe("aac");
@@ -105,7 +105,7 @@ describe("buildPadTrimAudioArgs", () => {
     expect(winPlan.operation).toBe("pad");
     const args = winPlan.steps[0]!.args;
     expect(args).toContain("-af");
-    expect(args[args.indexOf("-af") + 1]).toBe("apad,atrim=0:5.000000");
+    expect(args[args.indexOf("-af") + 1]).toBe("apad,asetpts=N/SR/TB,atrim=0:5.000000");
     expect(args.join(" ")).not.toContain("whole_dur");
   });
 });
diff --git a/packages/producer/src/services/render/audioPadTrim.ts b/packages/producer/src/services/render/audioPadTrim.ts
index ae98e985f9..d7b7d7278f 100644
--- a/packages/producer/src/services/render/audioPadTrim.ts
+++ b/packages/producer/src/services/render/audioPadTrim.ts
@@ -22,6 +22,7 @@
 import { spawn } from "node:child_process";
 import { rmSync } from "node:fs";
 import {
+  buildPadToDurationFilter,
   extractAudioMetadata,
   formatFfmpegError,
   getFfprobeBinary,
@@ -109,10 +110,11 @@ export interface PadTrimAudioPlan {
  * sequence that materializes it. Exported separately so unit tests can pin
  * every branch without spawning ffmpeg.
  *
- *   - `sourceDuration < targetDuration` → pad with `apad` to the exact target
- *     and re-encode AAC. This decodes and re-encodes the already mixed audio;
- *     an earlier concat-copy shape avoided that but could not produce a
- *     portable result on the bundled Windows FFmpeg builds.
+ *   - `sourceDuration < targetDuration` → pad to the exact target with the
+ *     shared `buildPadToDurationFilter` chain and re-encode AAC. This decodes
+ *     and re-encodes the already mixed audio; an earlier concat-copy shape
+ *     avoided that but could not produce a portable result on the bundled
+ *     Windows FFmpeg builds.
  *   - `sourceDuration > targetDuration` → filter to the exact target and
  *     re-encode AAC so packet padding cannot outlast the video.
  *   - `|Δ| < AUDIO_DURATION_TOLERANCE_SECONDS` → no-op `copy`, but we still
@@ -143,7 +145,7 @@ export function buildPadTrimAudioPlan(
             "-i",
             audioPath,
             "-af",
-            `apad,atrim=0:${targetSec}`,
+            buildPadToDurationFilter(targetSec),
             "-t",
             targetSec,
             "-c:a",
@@ -170,6 +172,10 @@ export function buildPadTrimAudioPlan(
           "-i",
           audioPath,
           "-af",
+          // `PTS-STARTPTS` here, not the pad branch's `N/SR/TB`: this branch
+          // only has to rebase an already-bounded stream to zero, while the
+          // pad branch has to rebuild a timeline an indefinite `apad` made
+          // untrustworthy for the `atrim` that follows it.
           `atrim=duration=${targetSec},asetpts=PTS-STARTPTS`,
           // `atrim` limits decoded samples but does not cap muxer timestamps
           // introduced by encoder delay/flush.  The output duration contract