Use document.requestPictureInPictureWindow instead of window.requestPictureInPictureWindow

[beaufortfrancois]

I wonder if we should use document.requestPictureInPictureWindow() instead of window.requestPictureInPictureWindow().
exitPictureInPicture() is exposed in document, not window so +1 for consistency.

partial interface Document {
  Promise<PictureInPictureWindow> requestPictureInPictureWindow(PictureInPictureWindowOptions options);
  attribute EventHandler onenterpictureinpicture;
  attribute EventHandler onleavepictureinpicture;
};

What do you think @steimelchrome?

[steimelchrome]

For requestPictureInPictureWindow(), I think that makes sense to me. We were originally mirroring window.open() a bit more, but I think it's different enough that it's reasonable to expose in Document instead.

Regarding the events: on issues 4 and 5 there's another suggestion to move the events to the PictureInPictureWindow object instead. I'm not sure what makes more sense between Document and PictureInPictureWindow

[beaufortfrancois]

I wouldn't move them to the PictureInPictureWindow object as it would prevent external scripts, extensions, etc. to listen to PiP state changes.

[domenic]

Where to put these things is always sort of a tossup, between window/document/navigator.

If there is any principle at all, it's that APIs on document should act on "the Document itself", or at least stuff in it. So exitPictureInPicture() is about HTMLVideoElements inside the Document. I.e. it's about document.pictureInPictureElement. So from that perspective, there's not much justification for using Document for this new operation, which creates a new Window separate from the current Window/Document. But I'm not sure if any developers would actually get confused here.

I think moving the event handlers to document would have a bigger risk of developer confusion though. I think developers would expect enterpictureinpicture to fire on Document when document.pictureInPictureElement changes, similar to how fullscreenchange fires on Document when document.fullscreenElement changes. And from what I can understand that's not the case; the events are purely about the separate window. (Which is part of why it's weird to have them fire on the parent window!! You don't fire load and unload events on the parent window after doing window.open(), even if that would make it easier for external scripts, extensions, etc. to listen to window state changes.)

So on balance I feel like keeping things on window probably makes more sense.