[whatwg] Full Screen API Feedback

Jer Noble jer.noble at apple.com
Wed May 11 21:45:33 PDT 2011

On May 11, 2011, at 7:41 PM, Robert O'Callahan wrote:

> On Thu, May 12, 2011 at 6:27 AM, Jer Noble <jer.noble at apple.com> wrote:
> 1. Z-index as the primary means of elevating full screen elements to the foreground.
> The spec suggests that a full screen element is given a z-index of BIGNUM in order to cause the full screen element to be visible on top of the rest of page content.  The spec also notes that  "it is possible for a document to position content over an element with the :full-screen pseudo-class, for example if the :full-screen element is in a container with z-index not 'auto'."  In our testing, we have found that this caveat causes extreme rendering issues on many major video-serving websites, including Vimeo and Apple.com.
> Can you describe these issues in more detail?

Sure.  Here's what Vimeo looked like in full-screen mode: <http://i.imgur.com/Rl4Gp.png>.  And Apple.com: <http://i.imgur.com/71Glg.png>.  Each page has already placed the video element in a stacking context one way or another.  And so, even though the full screen element has a large z-index, many other elements of the page pop in front of it.

> In order to fix rendering under the new full-screen API to be on par with WebKit's existing full-screen support for video elements, we chose to add a new pseudo-class and associated style rule to forcibly reset z-index styles and other stacking-context styles.  This is of course not ideal, and we have only added this fix for full screen video elements.  This rendering "quirk" makes it much more difficult for authors to elevate a single element to full-screen mode without modifying styles on the rest of their page.
> Proposal: the current API proposal simply recommends a set of CSS styles.  The proposal should instead require that no other elements render above the current full-screen element and its children, and leave it up to implementers to achieve that requirement.  (E.g., WebKit may implement this by walking up the ancestors of the full-screen element disabling any styles which create stacking contexts.)
> This could have side effects observable to the page. I'd prefer to standardize exactly what happens here.

I agree that an explicit requirement is desirable.

> 2. Animating into and out of full screen.
> WebKit's current video full-screen support will animate an element between its full-screen and non-full-screen states.  This has both security and user experience benefits.  However, with the current z-index-based rendering technique recommended by the proposed Full Screen API, animating the full-screen transition is extremely difficult.
> Proposal: The full-screen element should create a new view, separate from its parent document's view.  This would allow the UA to resize and animate the view separate from the parent document's view. This would also solve issue 1 above.
> I'm not sure what you mean exactly by a "new view". Depending on what you mean, that could create all kinds of implementation and spec issues. For example, if an element can have different style or layout in the two views, DOM APIs that return those things become ambiguous. I would strongly object to that.

I'm not suggesting that the element exists in two views simultaneously, but rather that it becomes the root of a new viewport.

> It seems to me you could animate the transition without having multiple concurent views. For example, "freeze" the rendering of the document in its browser window, put the document into the fullscreen state, and display it in a popup window that starts off matching the geometry of the fullscreen element and zooms out to cover the screen.

That is much more difficult than it sounds.  :)

Freezing the non-full-screen content is already undesirable.  The animation can take an arbitrary amount of time to complete, an any animations or dynamic content will appear to "hang" until the animation completes or the dynamic content is obscured.  But you're right that it would be required in order for this technique to work at all.

I've managed to implement a full screen animation which allows the non-full-screen content to continue live during the full screen animation, but it relies on hardware acceleration and required a large number of intrusive changes to the rendering engine.  Creating a new viewport for the full-screen content would serve the same purpose, and it would solve the z-index issue as well.  Resizing the full-screen viewport wouldn't affect the layout of the non-full-screen content, allowing for efficient animation of just the full-screen element and its children.

> 3. "fullscreenchange" events and their targets.
> The current proposal states that a "fullscreenchange" event must be dispatched when a document enters or leaves full-screen. Additionally, "when the event is dispatched, if the document's current full-screen element is an element in the document, then the event target is that element, otherwise the event target is the document."  This has the side effect that, if an author adds an event listener for this event to an element, he will get notified when an element enters full screen, but never when that element exits full-screen (if the current full screen element is cleared, as it should be, before the event is dispatched.)  In addition, if the current full-screen element is changed while in full screen mode (e.g. by calling requestFullScreen() on a different element) then an event will be dispatched to only one of the two possible targets.
> Proposal: split the "fullscreenchange" events into two: "fullscreenentered" and "fullscreenexited" (or some variation thereof) and fire each at the appropriate element.
> That sounds fine to me.
> 4. A lack of rejection.
> The current proposal provides no notification to authors that a request to enter full screen has been denied.  From an UA implementor's perspective, it makes writing test cases much more difficult.  From an author's perspective it makes failing over to another full screen technique (such as a "full-window" substitute mode) impossible.
> Proposal: add a "fullscreenrequestdenied" event and require it to be dispatched when and if the UA denies a full-screen request.
> My main concern is that with some UI scenarios there might not be a good time to fire the "denied" event. For example, in Firefox 4 when an application requests geolocation a popup appears, and if the user clicks anywhere outside the popup the popup disappears but there is still UI allowing the user to grant the request later. If we used the same approach for fullscreen, I think we wouldn't want to fire the denied event unless the user actually selects "no" in the popup. (It would seem confusing to deny the request and then grant it later.) I'm wary of authors writing code that assumes a denied event will fire and breaks when it doesn't, or when it fires later than they expect.

The current API already requires that authors listen for events that may occur in the far future.  I don't see how this event would be any different. 

And your geolocation example actually argues the other way: the existing geolocation API includes an asynchronous error handler that is explicitly called when a request is denied.  This would be a similar if not identical use case.

> For your use-case of falling back to a "full-window" substitute mode, I would suggest Web authors automatically go into the full-window state almost immediately after requesting fullscreen, but cancel it if the window actually goes into fullscreen mode.

That seems non-optimal.  It would result in a very confusing user experience ("The page is requesting full screen?  But it already is full screen!"), and I doubt any authors would choose to implement it that way.


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