_onDocumentLoaded() {
    // Find all inert roots in document and make them actually inert.
    const inertElements = Array.from(this._document.querySelectorAll('[inert]'));
    for (let inertElement of inertElements)
      this.setInert(inertElement, true);

    // Comment this out to use programmatic API only.
    this._observer.observe(this._document.body, { attributes: true, subtree: true, childList: true });
  }
Callback used when document has finished loading.