Proposed HTML5 fullscreen change: don't exit fullscreen on blur

[Chris Pearce]

Hi all,

I'd like to change our HTML5 fullscreen implementation to not exit
fullscreen when the browser window loses focus.

The reason we exit fullscreen when a fullscreen browser window loses
focus is to guard against a malicious page masquerading as the users
desktop when it loses focus. This threat was proposed back in the first
fullscreen security review:
https://wiki.mozilla.org/Security/Reviews/Firefox10/CodeEditor/FullScreenAPI

To mitigate this threat I propose that we show the "$domain.com is now
fullscreen" warning for 4 seconds when the fullscreen window which has
lost focus is re-focused. This is the same warning that is temporarily
shown (and for the same duration) when you enter fullscreen on a domain
which has previously been approved for fullscreen with the option to
remember the approval for that domain checked.

See my blog post at for an overview of the current UI to approve fullscreen:
http://blog.pearce.org.nz/2012/05/improved-key-input-in-fullscreen-mode.html

You can play with fullscreen in release Firefox on my unexciting test
page at:
http://pearce.org.nz/fullscreen/

I want to make this change to support the following use cases:

1. Watching fullscreen video in fullscreen on one monitor while
continuing to work on another (bug 724554).
2. Many function keys on Linux switch focus to a new window to show
some UI before switching back to Firefox's fullscreen window. For
example when you press the Volume Up/Down keys on Linux, most window
managers momentarily switch focus to a volume meter pop up window,
and this loss of focus causes Firefox to exit fullscreen (bug 578265
and its duplicates).
3. Allowing multiple fullscreen windows on multiple monitors. Authors
of immersive experiences (for example "The wilderness downtown" and
ro.me demos) want fullscreen windows covering all monitors to
increase the immersiveness of their experience.

I've had quite a number of people request this change, our behaviour of
exiting fullscreen on blur is very unpopular.

Please advise me if you have any concerns with me making this change.

Regards,
Chris Pearce.