_overlayTemplate() {
    // TODO: add performance optimization to only render the calendar if needed
    // When not opened (usually on init), it does not need to be rendered.
    // This would make first paint quicker
    return html`
      <div id="overlay-content-node-wrapper">
        ${this._overlayFrameTemplate()} ${this._arrowNodeTemplate()}
      </div>
    `;
  }
Defining this overlay as a templates from OverlayMixin
this is our source to give as .contentNode to OverlayController.
Important: do not change the name of this method.
@protected