async open(dialog) {
    if (!this.#overlays.has(dialog)) {
      throw new Error("The overlay does not exist.");
    } else if (this.#active) {
      if (this.#active === dialog) {
        throw new Error("The overlay is already active.");
      } else if (this.#overlays.get(dialog).canForceClose) {
        await this.close();
      } else {
        throw new Error("Another overlay is currently active.");
      }
    }
    this.#active = dialog;
    dialog.showModal();
  }
@param {HTMLDialogElement} dialog - The overlay's DOM element.
@returns {Promise} A promise that is resolved when the overlay has been
                   opened.