isplayed at once.

  See the [demo source code](https://github.com/PolymerElements/iron-overlay-behavior/blob/master/demo/simple-overlay.html)
  for an example.

  ### Closing and canceling

  An overlay may be hidden by closing or canceling. The difference between close and cancel is user
  intent. Closing generally implies that the user acknowledged the content on the overlay. By default,
  it will cancel whenever the user taps outside it or presses the escape key. This behavior is
  configurable with the