replace Shadow DOM support

[rodneyrehm]

>>>, /deep/ and ::shadow have been deprecated, see https://www.chromestatus.com/features/6750456638341120

We can traverse the DOM ourselves using Sizzle (probably overkill) or TreeWalker

[rodneyrehm]

well, this will find all ShadowHost elements:

// see https://developer.mozilla.org/en-US/docs/Web/API/NodeFilter
var ShadowHostFilter = {
  acceptNode: function(node) {
    return node.shadowRoot ? NodeFilter.FILTER_ACCEPT : NodeFilter.FILTER_SKIP;
    // possible optimization using FILTER_REJECT to skip checking the children
  }
};

function findShadowHosts(context, deep) {
  // see https://developer.mozilla.org/en-US/docs/Web/API/Document/createTreeWalker
  var shadowHostFinder = document.createTreeWalker(
    // root element to start search in
    context || document.documentElement,
    // element type filter
    NodeFilter.SHOW_ELEMENT,
    // custom NodeFilter filter
    ShadowHostFilter
  );

  var shadowHosts = [];
  while (shadowHostFinder.nextNode()) {
    shadowHosts.push(shadowHostFinder.currentNode);
    if (deep) {
      var children = findShadowHosts(shadowHostFinder.currentNode.shadowRoot, deep);
      shadowHosts = shadowHosts.concat(children);
    }
  }

  return shadowHosts;
}

// find all ShadowHosts recursively
findShadowHosts(document.documentElement, true);

[rodneyrehm]

Maybe this was a bit premature, as >>> will remain available to querySelectorAll() - https://twitter.com/tabatkins/status/624254124398243840

We'll have to figure out how to make ally/style/focus-source work without >>>.

[rodneyrehm]

query/focusable in "quick" mode (default) requires the support of "Shadow Piercing Descendant Combinator" for Shadow DOM introspection, "strict" mode does not (and will therefore work in Firefox).