nextcloud/php/public/second-tab-warning.js

// SPDX-FileCopyrightText: 2022 Nextcloud GmbH <https://nextcloud.com>
// SPDX-License-Identifier: AGPL-3.0-only

const channel = new BroadcastChannel('tab')

channel.postMessage('second-tab')
// note that listener is added after posting the message

channel.addEventListener('message', (msg) => {
    if (msg.data === 'second-tab') {
        // message received from 2nd tab
        document.getElementById('overlay').classList.add('loading')
        alert('Cannot open multiple instances. You can use AIO here by reloading the page.')
    }
});