third_party/polymer/components-chromium/paper-menu-button/paper-menu-button-transition-extracted.js

    Polymer('paper-menu-button-transition', {

      baseClass: 'paper-menu-button-transition',
      revealedClass: 'paper-menu-button-revealed',
      openedClass: 'paper-menu-button-opened',
      closedClass: 'paper-menu-button-closed',

      duration: 500,

      setup: function(node) {
        this.super(arguments);

        var bg = node.querySelector('.paper-menu-button-overlay-bg');
        bg.style.transformOrigin = this.transformOrigin;
        bg.style.webkitTransformOrigin = this.transformOrigin;
      },

      transitionOpened: function(node, opened) {
        this.super(arguments);

        if (opened) {
          if (this.player) {
            this.player.cancel();
          }

          var anims = [];

          var ink = node.querySelector('.paper-menu-button-overlay-ink');
          var offset = 40 / Math.max(node.cachedSize.width, node.cachedSize.height);
          anims.push(new Animation(ink, [{
            'opacity': 0.9,
            'transform': 'scale(0)',
          }, {
            'opacity': 0.9,
            'transform': 'scale(1)'
          }], {
            duration: this.duration * offset
          }));

          var bg = node.querySelector('.paper-menu-button-overlay-bg');
          anims.push(new Animation(bg, [{
            'opacity': 0.9,
            'transform': 'scale(' + 40 / node.cachedSize.width + ',' + 40 / node.cachedSize.height + ')',
          }, {
            'opacity': 1,
            'transform': 'scale(0.95, 0.5)'
          }, {
            'opacity': 1,
            'transform': 'scale(1, 1)'
          }], {
            delay: this.duration * offset,
            duration: this.duration * (1 - offset),
            fill: 'forwards'
          }));

          var nodes = window.ShadowDOMPolyfill ? Platform.queryAllShadows(node.querySelector('core-menu'), 'content').getDistributedNodes() : node.querySelector('core-menu::shadow content').getDistributedNodes().array();
          var items = nodes.filter(function(n) {
            return n.nodeType === Node.ELEMENT_NODE;
          });
          var itemDelay = offset + (1 - offset) / 2;
          var itemDuration = this.duration * (1 - itemDelay) / items.length;
          items.forEach(function(item, i) {
            anims.push(new Animation(item, [{
              'opacity': 0
            }, {
              'opacity': 1
            }], {
              delay: this.duration * itemDelay + itemDuration * i,
              duration: itemDuration,
              fill: 'both'
            }));
          }.bind(this));

          var shadow = node.querySelector('paper-shadow');
          anims.push(new Animation(shadow, function(t, target) {
            if (t > offset * 2 && shadow.z === 0) {
              shadow.z = 1
            }
          }, {
            duration: this.duration
          }));

          var group = new AnimationGroup(anims, {
            easing: 'cubic-bezier(0.4, 0, 0.2, 1)'
          });
          this.player = document.timeline.play(group);
        }
      },

    });