[cssom-view-1] Define HTMLElement.scrollParent

[flackr]

Define the scrollParent of an element following the discussion and resolution in #1522.

[tabatkins]

Hm, it's slightly unfortunate that a null can mean either that the scroll parent is the root scroller, or that the scroll parent is hidden in a closed shadow. The former can still be responded to, by scrolling the root, the latter can't be, and I'm not sure there's an easy way to tell them apart.

Is that okay? Or do we want to be able to distinguish those cases?

Edit: Well I guess the third case is that it's a fixpos, which skips all the scrollers anyway and just attaches to the viewport, and thus isn't scrollable either. Hm.

[flackr]

The former can still be responded to, by scrolling the root, the latter can't be, and I'm not sure there's an easy way to tell them apart.

Right, I could see a good argument for skipping the hidden parent. WDYT of just having it keep walking up the containing block chain in that case? I think you're right that scrolling the next scroller up should still provide reasonable behavior. I've updated the steps accordingly. I think skipping the hidden ancestors is similar to offsetParent actually so this just seems generally better.