Gecko:FullScreenAPI

Requirements

• Scripts should be able to initiate and terminate full-screen
  • Allows custom, in-page, discoverable UI to active full-screen
• Arbitrary Web content should be visible full-screen
  • <video>, <canvas>, multicolumn text, etc
  • Required for scripted video controls
• Entering and exiting full-screen must trigger events
  • To allow scripted changes to content, e.g. resizing of canvas content
• Scripts should be able to opt in to having alphanumeric keyboard input disabled during full-screen
  • Enables less restrictive anti-spoofing measures
• Actual full-screen transitions must be allowed to be asynchronous and entirely at the discretion of the UA
  • To enable passive notification/confirmation UI
  • To enable a wide range of security measures to prevent spoofing or other abuse

• Making a single element full-screen should be as simple as possible

• Content in IFRAMEs should be able go full-screen
  • To enable "widgets" such as embedded videos to offer full-screen UI

Proposed Specification

A Document can be in the "full-screen state". What exactly this means is up to the UA, but typically it means that the Document covers most or all of the screen and some or all of the normal UA UI is not visible.

It is possible for non-toplevel browsing contexts to have their Document in the full-screen state. The parent browsing context of a browsing context with its Document in the full-screen state must also have its Document in the full-screen state.

The user agent may transition a Document into or out of the full-screen state at any time, whether or not script has requested it. User agents are encouraged to provide standard UI to exit the full-screen state, for example if the user presses the Escape key.

Toplevel browsing contexts can be in a "keys disabled" state. In this state, the user agent must suppress all keyup, keydown and keypress events whose keyCode is not in one of the following ranges:

• DOM_VK_CANCEL to DOM_VK_CAPS_LOCK, inclusive
• DOM_VK_SPACE to DOM_VK_DELETE, inclusive
• DOM_VK_SEMICOLON to DOM_VK_EQUALS, inclusive
• DOM_VK_MULTIPLY to DOM_VK_META, inclusive

Such events are not dispatched to any nodes in any document of the toplevel browsing context or descendant browsing contexts. This includes suppression of any internal key event processing that would insert text into form controls or editable content. The user agent might respond to such events by leaving full-screen mode.

Each document has an optional "current full-screen element".

New method of Document:

• void cancelFullScreen()

Requests that the UA exit full-screen mode. The UA is not required to honor this, for example the UA might require that only a Document that last triggered full-screen can cancel it.

The current full-screen element for the document is cleared.

New methods of Element:

• void requestFullScreenWithKeys()

The current full-screen element for the document is set to this element.

Typically the user agent would react by transitioning the Document to the full-screen state, or by presenting asynchronous confirmation UI and transitioning to the full-screen state if/when the user responds affirmatively. However, the user agent is not required to do anything at all in response to requestFullScreen. The user agent's behavior is allowed to vary depending on whether requestFullScreen is called during a user event (e.g. a mouse click handler).

• void requestFullScreen()

As requestFullScreenWithKeys, but hints to the UA that while in full-screen state, the toplevel browsing context for this Document should have keys disabled. While keys are disabled, there may be a reduced risk of spoofing attacks inducing the user to input inappropriate data, and the UA may choose to relax restrictions on entering full-screen state with keys disabled.

New DOM attribute of Document:

• readonly attribute boolean fullScreen

Returns true while the document is in the full-screen state.

• readonly attribute boolean fullScreenKeyboardInputAllowed

Returns true while the window's toplevel browsing context is full-screen and not in a "keys disabled" state.

New content attribute of the <iframe> element:

• allowfullscreen

This is a boolean attribute. When this attribute is not set, UAs must ignore full-screen requests in the iframe or its descendant frames.

New events:

• fullscreenchange

When a Document enters or leaves the full-screen state, the user agent must queue a task to dispatch this event. 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. The event bubbles and is not cancellable.

The 'onfullscreenchange' event handling attribute is supported on HTML elements.

New CSS psuedoclass:

• :full-screen

While a Document is in the full-screen state, and the document's current full-screen element is an element in the document, the 'full-screen' pseudoclass applies to that element. Also, an <iframe>, <object> or <embed> element whose child browsing context's Document is in the full-screen state has the 'full-screen' pseudoclass applied.

New CSS media query selector:

• full-screen

that accepts two states:

• on
• off

While a Document is in the full-screen state, the full-screen media type is active for the document.

Suggested UA stylesheet rules:

/* A full-screen element that is not the root element should be stretched
   to cover the viewport. */
:full-screen:not(:root) {
  position:fixed;
  top:0;
  left:0;
  right:0;
  bottom:0;
  z-index:2147483647;
  background:black;
}
/* In full-screen mode, if the root element itself is not full-screen then
   we should hide the viewport scrollbar. */
@media all and (full-screen:on) {
  :root:not(:full-screen) {
    overflow:hidden;
  }
}

Note 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'.

Suggested UA Policy

The specification intentionally gives UAs great freedom in policy, because no one policy can fit all users, devices, and user interface designs. However, here is a policy that should be acceptable for conventional desktop browsers.

• requestFullScreen while the window is already in the full-screen state is approved.
• Otherwise, requestFullScreen outside a user action (e.g. a non-synthesized input event handler) is denied.
• Otherwise, requestFullScreen without the ALLOW_KEYBOARD_INPUT flag is approved.
• Otherwise, passive confirmation UI is presented and requestFullScreen is approved if and when the user approves it.

Implications

There is no requirement to exit full-screen state when a browsing context is navigated to a new page.

There is no requirement to exit full-screen state if the current full-screen element shifts to another element, or ceases to be an element in the document (e.g. because requestFullScreen was called again in the document or in some descendant document, or because the element is removed).

Nothing in this specification depends on whether the current full-screen element is visible (e.g. whether it has "display:none").

Nothing in this specification affects or depends on focus.

Examples

1. Make a video element display full-screen when clicked and leave full-screen when it ends.

<video src="pelican.webm" autoplay
       onclick="event.target.requestFullScreen()"
       onended="document.cancelFullScreen()">
</video>

2. Make a canvas element display full-screen in response to user input. Resize the canvas to the appropriate resolution while it's full-screen.

<script>
function redrawCanvas(c) { ... }
function resizeCanvas() {
  var c = document.getElementById("c");
  var rect = c.getBoundingClientRect();
  c.width = rect.width;
  c.height = rect.height;
  redrawCanvas(c);
}
</script>
<canvas id="c" onfullscreenchange="resizeCanvas()"></canvas>
<button onclick="document.getElementById('c').requestFullScreen()">
  Go Fullscreen!
</button>

3. Hide advertisements while the window is full-screen.

@media all and (full-screen:on) {
  .advertisement { display:none; }
}

Security

Date of discussion: 2011.04.11

Security Concerns:

• Ability of website to enter fullscreen and pre-empt keyboard focus
• User interaction currently not required for entering full screen mode
• Fullscreen could be used as an attack vector

Responses:

• There is a mode called without keys that does not take keyboard input
• Focus is released on tab change or window change

Possible Remediations:

• ESC key should be used to exit, similar to other well known apps users are familiar with
• A user preference should be available for users to say allow full-screen or dis-allow full screen for a given URL domain (Ie. Popup or geolocation preferences)
• Possible use of some indicator to show a user they are in full-screen mode
• Possible use of permission manager
• Plug-ins should be disabled when in full-screen mode

To-Do

• Re-review as spec firms up and code begins to land

Issues

naming one word or two

fullscreen vs full screen

• fullscreen
  • text of this page repeatedly uses "fullscreen"
  • Wikipedia prefers "fullscreen" and redirects "full screen" to that page.

• full screen
  • title of this page implies "full screen" from the camelcase: (FullScreenAPI)
  • the Firefox 4 "View" menu item "Full Screen" (shift-command-F)

roc: Elika and I resolved to use 'full-screen' everywhere.

avoiding ancestor reflow

Currently, the suggested UA stylesheet rules for a non-root fullscreen element take it out of the flow and make it position fixed, which has the side-effect of causing its ancestors to reflow, which is unnecessary, and thus an undesirable performance hit.

Instead, we should consider simply defining the fullscreened element as creating a new view with itself as the root box of a CSS presentation, with its own opaque CSS canvas (as defined in CSS), thereby avoiding the reflow problem.

The underlying background of this new opaque CSS canvas comes from the fullscreened element, just as the underlying background of a window comes from the root element of the document in that window.

roc: I don't want to do it this way. Creating additional views is very difficult to fit into the architecture of Gecko and other engines. The existing way seems fine in practice. Reflowing the document is not going to be a significant performance issue.

avoiding drawing anything else

Also, the suggested UA stylesheet rules use z-index to put the fullscreened element above everything else. Instead we may want to specify that when something is fullscreened, that we don't bother rendering any other windows/tabs, nor the parent document.

roc: Again this is more invasive to the engine than necessary. The z-index approach should work fine and is simple, without requiring any special engine support.