Allow gUM prompt ahead of focus + deterministic "visible" enumeration wo/focus

[jan-ivar]

Fixes #752:

1. Relax document (iframe) focus to tlbc system focus (a step in the right direction)
2. Allow Safari prompts in foreground tabs, while maintaining focus requirement on allow, per Broken foreground detection #752 (comment)
3. Remove focus requirement on enumerateDevices, per Broken foreground detection #752 (comment)
4. Fix race spotted in Why does navigator.mediaDevices.enumerateDevices() require that Document must have active keyboard focus? #905 (comment) which prevented deterministic visibility-success checks:

   if (document.visibilityState == "visible") {
     await navigator.mediaDevices.enumerateDevices(); // won't block on hidden thanks to if
   }

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Preview | Diff

[eladalon1983]

Generally LGTM.

[eladalon1983]

The prose here was difficult for me to process. How about simplifying it with something along the lines of "...queue a task to do X. Once the task resolves, proceed to the next steps."

[jan-ivar]

I'm open to simpler ways to say it, but queued tasks don't "resolve" AFAIK. Also, waiting for the task alone is not enough, since that might proceed on false. The intent here is to describe the same criteria as before without the cross-thread access.

[jan-ivar]

I've done a tweak with s/If/While/ combined with broken out algorithms that are hopefully easier on the eyes. PTAL.

[eladalon1983]

As a follow up, I wonder if we could refine our concept of focus to accommodate the possibility that UA-level elements might override the app's focus even when visible, e.g. if the user is interacting with DevTools, the omnibar, bookmarks drop-down lists, or any future UA-level element of arbitrary complexity which could be prominently displayed and commanding the user's attention.

[jan-ivar]

Yes, that's whatwg/html#6211, which is hopefully not a blocker for this PR which moves us a step closer.

[alvestrand]

@guidou is this deployable?

[guidou]

@guidou is this deployable?

I think this is an improvement over the focus requirement. In terms of deployability, we'll have to implement and evaluate it. The focus requirement introduced serious regressions for many of Chromium users and we had to roll it back.

[guidou]

Does this mean we still need to have system focus?

[jan-ivar]

In getUserMedia, yes. The focus requirement here is just moved down to allow prompting ahead of focus, like Safari does.

[alvestrand]

This seems to be a more deployable change than the change from present behavior that is currently in the spec, so it's an improvement. We'll see if it can deploy.