function containsDeep(parent, child) {
    var node = child
    while (node) {
      if (node == parent) return true

      node = getParentNode(node)
    }
    return false
  }
Checks to see if a parent element contains a child element (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.