navigator.sendBeacon does not work on visibilitychange callback for document unload

[Bastien Caudan]

With the example from the beacon spec (https://w3c.github.io/beacon/):

document.addEventListener('visibilitychange', function() {
  if (document.visibilityState === 'hidden') {
    navigator.sendBeacon('/collector', data);
  }
});


From manual tests on hidden cases, navigator.sendBeacon is triggered correctly when:
- the browser is minimized.
- the document is not the active tab

But navigator.sendBeacon is not triggered when:
- the browser unload the document

Other browsers that support the Beacon API allow these requests to be sent.

[Radar WebKit Bug Importer]

<rdar://problem/48301429>

[Daniel]

The visibilitychange event don’t fire at all during unload. This is a duplicate of issue #116769.

[sideshowbarker]

See the https://volument.com/blog/sendbeacon-is-broken#comments comments discussion for some real-world pain points developers are hitting due to this. (So that not only do we need to make developers aware that they shouldn’t be trying to use unload/onunload with sendBeacon(), and that they instead need to  visibilitychange — but *also* need to check pagehide (because we don’t yet have visibilitychange working interoperably for them across the web platform).

[Sam Sneddon [:gsnedders]]

*** This bug has been marked as a duplicate of bug 151234 ***