Element should stop scrolling with viewport at the bottom of its parent container

[nring]

This functionality might already be in the polyfill, but I was unable to get it working.

As the parent element of an element with position:sticky; approaches the top of the browser viewport (as the user scrolls down), the `position:sticky' element should stop its 'sticky' behavior as the parent element goes out of the viewport.

In the HTML5Rocks demo page, the blue blog headers stop at the bottom of their parent elements.

[philipwalton]

Yeah, I'm aware of this limitation in the current example. Eventually I'd like to improve it, but at this point it was mainly for demonstration purposes only.

Also, I don't believe there's an official spec, so it's hard to say what the "intended" behavior is in that case.