gum_video_button_beforeunload.html

<html>
<meta charset=utf-8>
<div id="div"></div>
<button id="button">Camera</button>
<button id="trackstop">Stop</button>
<button id="reload">Refresh page</button>
<a href="https://mozilla.github.io/webrtc-landing/enumerate.html">Navigate</a><br>
<video id="video" width="320" height="200" autoplay></video><br>
<script>
const console = { log: msg => div.innerHTML += msg + "<br>" };

button.onclick = async () => {
  try {
    video.srcObject = await navigator.mediaDevices.getUserMedia({video: true});
  } catch(e) {
    console.log(e);
  }
};

trackstop.onclick = () => {
  for (const track of video.srcObject.getTracks()) track.stop();
  video.srcObject = null;
}
reload.onclick = () => location.reload();

const beforeUnloadListener = (event) => {
  event.preventDefault();
  return event.returnValue = "Are you sure you want to exit?";
};

window.onbeforeunload = event => {
  event.preventDefault();
  return event.returnValue = "Are you sure you want to exit?";
};
window.onunload = () => {
  navigator.mediaDevices.getUserMedia({video: true}).catch(() => {});
  trackstop.onclick();
}

</script>
</html>