function sortByDOMPosition( a, b ) {
  return a.node.compareDocumentPosition( b.node ) === 10 ? 1 : -1;
}
sortByDOMPosition: Called by our click handler to sort a list of instances
according to least -> most nested. This is so that if multiple keybound
instances have nodes that are ancestors of the click target, they will be
sorted to let the instance closest to the click target get first dibs on the
next key down event.