Intent to implement and ship: document.scrollingElement

[Boris Zbarsky]

Summary: A property that makes it possible for web pages to tell which
element's scroll* attributes reflect the viewport scroll state. This is
needed because currently web pages have different codepaths (using
document.body vs document.documentElement) for different browsers based
on UA sniffing, which keeps Blink and WebKit from switching to our (and
the standard's) behavior. With this property the page could have a
single codepath using document.scrollingElement and ditch the UA
sniffing and then Blink/WebKit could switch their behavior, as long as
they change both scroll* and .scrollingElement at the same time.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1153322 which has
links to some of the other discussion.

Spec: http://dev.w3.org/csswg/cssom-view/#dom-document-scrollingelement

Platforms: All of them.

Preference: none.

DevTools bug: Don't think this needs devtools work.

Support in other UAs: Chrome is actively working on this, so they can
update to the spec for scroll* sooner rather than later.

Target release: Gecko 40.

-Boris