Widget should determine its own clippedBy

[ExtAnimal]

clippedBy is a hidden Widget config which tells a floating widget which scrolling elements create the bounding viewport which clips its alignTo target.

This is how tooltips know when their target has been scrolled out of view, and how they align to the visible center of the target.

This should be calculable at alignTo time when the align target is an HTMLElement:

I suggest a new static Scroller method:

/**
 * Returns the scrolling ancestors which provide the clipping viewport for the passed element.
 */
static getClippedBy(target) {
    const
        clippedBy = [],
        v         = target.ownerDocument.defaultView;

    for (let el = target; el != document; el = el.parentElement) {
        const s = v.getComputedStyle(el);

        if (isScrollable[s.overflowX] || isScrollable[s.overflowY]) {
            clippedBy.push(el);
        }
    }
    return clippedBy;
}