paper-dropdown-transition.css

/* Copyright (c) 2014 The Polymer Project Authors. All rights reserved.
This code may only be used under the BSD style license found at http://polymer.github.io/LICENSE.txt
The complete set of authors may be found at http://polymer.github.io/AUTHORS.txt
The complete set of contributors may be found at http://polymer.github.io/CONTRIBUTORS.txt
Code distributed by Google as part of the polymer project is also
subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt */

:host(.core-transition) #ripple,
:host(.core-transition) #background {
  opacity: 0;
}

:host(.core-transition) #scroller {
  opacity: 0;
}