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diff --git a/dom/media/test/test_setSinkId_after_loop.html b/dom/media/test/test_setSinkId_after_loop.html
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+++ b/dom/media/test/test_setSinkId_after_loop.html
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+<!DOCTYPE HTML>
+<html>
+<head>
+<title>Test setSinkId() after looping</title>
+<script src="/tests/SimpleTest/SimpleTest.js"></script>
+<link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
+</head>
+<script type="application/javascript">
+"use strict";
+
+/**
+This test captures loopback device audio to test that a media element produces
+audio output soon after a setSinkId() call after looping.  In bug 1838756,
+incorrect timekeeping caused silence in the output as long as the duration of
+loops completed.
+ **/
+add_task(async () => {
+  const audio = new Audio();
+  audio.src = "sin-441-1s-44100.flac";
+  audio.loop = true;
+  audio.controls = true;
+  // Mute initially so that delayed audio from before setSinkId does not
+  // trigger a false pass.
+  audio.muted = true;
+  document.body.appendChild(audio);
+  const canplaythroughPromise = new Promise(r => audio.oncanplaythrough = r);
+  // Let the audio loop at least twice, which would result in 2 extra seconds
+  // of silence with bug 1838756.
+  let count = 0;
+  const loopedPromise = new Promise(r => audio.onseeked = () => {
+    if (++count == 2) {
+      r();
+    }
+  });
+
+  await SpecialPowers.pushPrefEnv({set: [
+    // skip gUM permission prompt
+    ["media.navigator.permission.disabled", true],
+    // enumerateDevices() without focus
+    ["media.devices.unfocused.enabled", true],
+  ]});
+  let loopbackInputLabel =
+      SpecialPowers.getCharPref("media.audio_loopback_dev", "");
+  if (!navigator.userAgent.includes("Linux")) {
+    todo_isnot(loopbackInputLabel, "", "audio_loopback_dev");
+    return;
+  }
+  isnot(loopbackInputLabel, "",
+        "audio_loopback_dev. Try --use-test-media-devices.");
+  const loopbackStream =
+        await navigator.mediaDevices.getUserMedia({audio: true});
+  is(loopbackStream.getTracks()[0].label, loopbackInputLabel,
+     "loopback track label");
+
+  await canplaythroughPromise;
+  let loopbackNode;
+  try {
+    audio.play();
+    await new Promise(r => audio.onplaying = r);
+    const ac = new AudioContext;
+    loopbackNode = ac.createMediaStreamSource(loopbackStream);
+    const processor1 = ac.createScriptProcessor(4096, 1, 0);
+    loopbackNode.connect(processor1);
+
+    // Check that the loopback stream contains silence now.
+    const {inputBuffer} = await new Promise(r => processor1.onaudioprocess = r);
+    loopbackNode.disconnect();
+    is(inputBuffer.getChannelData(0).find(value => value != 0.0), undefined,
+       "should have silence");
+
+    // Find output device
+    const devices = await navigator.mediaDevices.enumerateDevices();
+    let loopbackOutputLabel =
+        SpecialPowers.getCharPref("media.cubeb.output_device", "");
+    const outputDeviceInfo = devices.find(
+      ({kind, label}) => kind == "audiooutput" && label == loopbackOutputLabel
+    );
+    ok(outputDeviceInfo, `found ${loopbackOutputLabel}`);
+
+    await loopedPromise;
+    await audio.setSinkId(outputDeviceInfo.deviceId);
+    audio.muted = false;
+    // Use a new ScriptProcessor so that the first audioprocess event provides
+    // the rendering thread time after unmuting.
+    const processor2 = ac.createScriptProcessor(4096, 1, 0);
+    loopbackNode.connect(processor2);
+
+    let seenAudioCount = 0;
+    let lastSample, firstTime;
+    while (seenAudioCount < ac.sampleRate) {
+      const event = await new Promise(r => processor2.onaudioprocess = r);
+      let samples = event.inputBuffer.getChannelData(0);
+      for (const sample of samples) {
+        if (!seenAudioCount) {
+          if (!firstTime) {
+            firstTime = event.playbackTime;
+          }
+          if (sample != 0.0) {
+            seenAudioCount = 1;
+          } else {
+            const delay = event.playbackTime - firstTime;
+            if (delay > 1.0) {
+              ok(false, `${delay} seconds passed without receiving audio`);
+              return;
+            }
+          }
+        } else if (sample != 0.0) {
+          ++seenAudioCount;
+        } else if (lastSample == 0.0) {
+          ok(false, "should not have two consecutive zero sample in audio");
+          return;
+        }
+        lastSample = sample;
+      }
+    }
+    ok(true, "have one continuous second of audio");
+  } finally {
+    if (loopbackNode) {
+      loopbackNode.disconnect();
+    }
+    audio.pause();
+    loopbackStream.getTracks()[0].stop();
+  }
+});
+</script>
+</html>