code: function containsDeep(parent, child) {
          var node = child;
          while (node) {
              if (node == parent) return true;
  
              node = getParentNode(node);
          }
          return false;
      }
docs: Checks to see if a parent element contains a child elemnt (including inside
  shadow DOM).
  @param {Node} parent The parent element.
  @param {Node} child The child element.
  @return {boolean} True if the parent node contains the child node.