A use for adoptedCallback

adoptedCallback.js

adoptedCallback(oldDocument, newDocument) {
  const newWindow = newDocument.defaultView;
  if (newWindow) {
    // newDocument belongs to a window
    const otherConstructor = newWindow.customElements.get(this.localName);
    
    if (otherConstructor && otherConstructor._isFromPolymer) {
      Object.setPrototypeOf(this, otherConstructor.prototype);
      
      // Now any customizations that newWindow code has applied to the
      // prototype will apply over here.
    }
  }
}

Have you done any testing across compliant browsers?

That would be amazing for both incremental upgrades and hot-reloading (two pain points of SWC's)