lib/helpers-dom.js
function containerOf(a, b) {
  while (b = b.parent) {
    if (a === b) return true;
  }
  return false;
}
function containerOf(a, b) {
  while (b = b.parent) {
    if (a === b) return true;
  }
  return false;
}
Checks if container `a` is a container of `b`