feat: document fullscreenElement, pictureInPictureElement and visibilityState binding

[ThaUnknown]

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This PR adds binding for document's fullscreenElement, visibilityState and pictureInPictureElement, they are all however read only. This is useful for conditional UI which shows different things depending on fullscreen state etc

Considerations:

1. IN THEORY the properties could be writable, but that wouldn't operate on the document element but the element that's being written aka

fullscreenElement = document.querySelector('div')

would actually be

document.querySelector('div').requestFullscreen()

but I don't think that's a good idea, so I'm leaving the properties read only.

2. pictureInPictureElement refers only to the video PiP element, and doesn't handle the new Document Picture-in-Picture API as it uses window.documentPictureInPicture.window [????] instead of document.pictureInPictureElement to report what element is active as the current PiP window, questionable decision imho

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[ThaUnknown]

unrelated tests fail?

[dummdidumm]

Thank you!

Since pictureInPictureElement is not standardized (I think?) and since it sounds confusing in behavior, I suggest to just remove it from the PR - would that be ok?

[ThaUnknown]

Since pictureInPictureElement is not standardized

it is standardized, see this, just not implemented by firefox, since they added their own implementation for it [for some reason], even safari/opera supports it

it sounds confusing in behavior

why? it behaves identically to fullscreenElement, like actually 1:1, I'd much prefer to keep it, again, especially for conditional UI like displaying a different button when PiP is active, or hiding a video element when a custom PiP implementation is used, I originally made this PR for the PiP element, not fullscreen/visibilityState, so I'd prefer if we could keep it in

[dummdidumm]

Is it a standard though? It's an editor's draft, not a recommendation, which I think means it's not officially standardized yet. I was also proposing to remove it because you said it's confusing that the new picture in picture API uses different things.

[ThaUnknown]

It's an editor's draft

my bad

the new document PiP API isn't standardised, and only supported by new versions of chromium, so that was more of a note of "not to be confused with"

[dummdidumm]

This is a working draft which still is not the recommended stage. I mean everyone except Firefox implemented it already anyway, so maybe it's fine.

[ThaUnknown]

yeah, that's usually as far as new things will go, I agree that PiP isn't the most uuuuh agreed on thing, but there's very little stopping svelte from adding support for it, which would be nice

[dummdidumm]

Since it's still in working draft and since the current TypeScript definitions of this repo also don't contain it, I removed the picture-in-picture binding. The rest looks good, so I'll merge - thank you!

[ThaUnknown]

that's insanely sad

[Conduitry]

The fullscreenElement and visibilityState bindings have been released in 3.59.0.