Rolling forward https://codereview.chromium.org/1155683008/ which was

third_party/polymer/v1_0/components-chromium/iron-resizable-behavior/iron-resizable-behavior-extracted.js

 
   /**
    * `IronResizableBehavior` is a behavior that can be used in Polymer elements to
    * coordinate the flow of resize events between "resizers" (elements that control the
    * size or hidden state of their children) and "resizables" (elements that need to be
    * notified when they are resized or un-hidden by their parents in order to take
    * action on their new measurements).
    * Elements that perform measurement should add the `IronResizableBehavior` behavior to
    * their element definition and listen for the `iron-resize` event on themselves.
    * This event will be fired when they become showing after having been hidden,
@@ skipping 105 matching lines @@
 
       if (this._interestedResizables.indexOf(target) === -1) {
         this._interestedResizables.push(target);
       }
 
       target.assignParentResizable(this);
 
       event.stopPropagation();
     }
   };