Sorry, clarification - it wasn't a High Sierra update, it was a Security Update for Sierra. (I don't think this makes a difference w.r.t. to this bug though, just want to correct what I wrote.)

Do you happen to know if the YouTube tab was the visible tab in the Chrome window?

Yes, the Youtube tab was the foreground tab in the top-most window for Chrome.

This sounds like a system integration bug: most likely Chrome thought it was visible even if it was hidden behind the login screen. When a tab is marked as hidden, we do not allow any playback unless playback already occurred but we do allow it when visible.

+ccameron@, +thakis as they may know where this should be triaged.

This is probably  issue 526271  /  issue 560446  where the first tab in a window is always foreground, so doesn't trigger the autoplay block that we have for background tabs.

[mac bug triage]

> I updated macOS to High Sierra at home and at the time had a Chrome window with a Youtube tab open. (I don't think it was playing at the time.) After the update had finished, the OS restarted and the macOS log in screen was shown.

Were you truly at the login screen after a reboot? I'm trying to understand how Chrome could be running at that point because you had not yet logged into your machine.

Yes, I was. I assume it restored my apps in the background?

Those apps need access to files in your account, so you need to be logged in somehow.

I'll see if I can repro.

I tried on my off-corp 10.13 machine - it boots up to the login screen, but nothing happens after that. Are you able to repro at will? Seems like the important part of your problem description is the restart rather than why the machine was restarted.

In order for your tab to start playing video your Mac has to be logged into your account (to access your user data dir and reload your tabs). That should not just happen behind the scenes. I was wondering if perhaps your machine was booting up and you did something to trigger the login screen (e.g. with hot corners).

Sorry, I've not had a chance to look at this more. I think I tried to repro with a simple reboot once and didn't seem to repro it.

I agree that it does sound weird that it loaded before I logged in, but I'm pretty sure this is exactly what was the case - i.e. there was an auto-logout due to the update and a reboot.

As an aside, I suppose this issue could also happen if you lock your screen while Chrome is starting/loading and restoring tabs? That might be an easier scenario to test for and I think the behavior there should also be "not autoplay Youtube" if the page load happens when screen is locked.