xamxam/app/public/assets/js/beforeunload-guard.js

/**
 * Beforeunload guard — prompts the user before navigating away from unsaved changes.
 *
 * Attach to any form with a data-beforeunload-guard attribute.
 * No effect when JavaScript is unavailable (form posts normally).
 */
(() => {
    const forms = document.querySelectorAll('form[data-beforeunload-guard]');
    if (!forms.length) return;

    let dirty = false;

    for (const form of forms) {
        form.addEventListener('input', () => { dirty = true; });
        form.addEventListener('change', () => { dirty = true; });
        form.addEventListener('submit', () => { dirty = false; });
    }

    window.addEventListener('beforeunload', (e) => {
        if (dirty) {
            e.preventDefault();
        }
    });
})();