Add Picture in Picture window to note on local playback devices

[chrisn]

For context, see w3c/picture-in-picture#191.

The Picture in Picture spec currently says:

The [Remote-Playback] specification defines a local playback device and a local playback state. For the purpose of Picture-in-Picture, the playback is local and regardless of whether it is played in page or in Picture-in-Picture.

Media WG would like to move this language to the Remote Playback API to avoid having a patch in the Picture in Picture spec.

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[anssiko]

Thank you. This is a follow up to our discussion in https://www.w3.org/2023/09/15-webscreens-minutes.html#t17

My expectation is https://w3c.github.io/picture-in-picture/#remote-playback will be removed as redundant when this PR is merged.

[chrisn]

My expectation is https://w3c.github.io/picture-in-picture/#remote-playback will be removed as redundant when this PR is merged.

That's right, yes, that's our next step.

[mfoltzgoogle]

• There are other spec errors for undefined terms (browsing context, etc.) that will prevent merging the PR.
• While the change itself is fine, the following sentence doesn't apply to picture-in-picture, because the transition to picture-in-picture is initiated by the user agent (either through browser UI, or the PiP API itself). So updating the note further would make it clearer.

[mfoltzgoogle]

Merged #154 to fix the xrefs to HTML.

[chrisn]

Oh, thank you! I'll update this PR with an alternative phrasing.

[chrisn]

So now I'm looking at local playback state as a better place to mention this, and it made me wonder about the interaction between Remote Playback and Picture in Picture. I don't think it's enough to simply say that PiP playback is considered local:

• What should happen to the PiP state if the user initiates remote playback while a PiP window is open?
• What should happen if the user tries to request PiP while the remote playback state is anything other than disconnected?

What do implementations currently do in these scenarios?

[mfoltzgoogle]

At first glance, it would be reasonable for the browser to support video remote playback while in PiP, or choose to exit PiP during remote playback, depending on form factor / usability / implementation complexity tradeoffs. I don't know that the spec needs to be opinionated about this.

Chrome supports the former, but I'll will need a bit of time to confirm.