Exit Picture In Picture Algorithm Overhaul

[theIDinside]

Adding this issue to track the document.exitPictureInPicture() request, as it needs the same kind of overhaul that the requestPictureInPicture() algorithm does (see #245)

I think it may be solved by just wrapping the whole thing inside a queued task. It still needs to probably be in-parallel, given that we don't really know/can't specify how long "closing the PIP window" will take, or in how many disjoint/asynchronous steps that actually happen inside the browser implementations. It could be 1, it could be many.

Edit: I will be adding things that need to change as I come across them, thinking about this algorithm.

[chrisn]

As part of this, we should update the unloading document cleanup steps in HTML to call to the Exit Picture In Picture algorithm.

[theIDinside]

When this algorithm gets updated, requestPictureInPicture algorithm may also needs updating 11.5.1 If {{pictureInPictureElement}} is not null, run the [=exit Picture-in-Picture algorithm=].. But that's if this algorithm changes to take input variables instead of just accessing stuff, see comment. (This may or may not change though, just left here for posterity)

[theIDinside]

Given that requestPictureInPicture will call exit algorithm steps (see #245 PR) the Queue a task to fire ..., which is step 3, must stop doing so.

Queuing a task is an abstraction to allow for DOM mutations, but it also has effects on ordering and timing. If we're already running on a queued task "in line", queuing another task is unnecessary. It also means that, for the request case, the ordering of the events will be:

• video1 requests & enters pip
• video 2 requests pip
• request algorithm sees that video1 is pip, exit pip for video1, queue a task to fire event
• video 2 enters pip and fires event

Which means that video2 sees it's enter event before video1's leave event is observed.

Suggestion: Let's be more exact with how and where we queue tasks. This algorithm should run in-line with no queuing of tasks and instead let that be the responsibility of the caller, to make sure it's wrapped appropriately.

[theIDinside]

The exit algorithm requires #245 to land first, as the changes to the exit algorithm requires its parallel steps to be queued on the picture in picture parallel queue.