stackable-modals.bootstrap.js

    /**
     * Make bootstrap modal dialogs stackable.
     */
    $(document).on('show.bs.modal', '.modal', function () {
        const zIndex = 1040 + (10 * $('.modal:visible').length);
        $(this).css('z-index', zIndex);
        setTimeout(function() {
            $('.modal-backdrop').not('.modal-stack').css('z-index', zIndex - 1).addClass('modal-stack');
        }, 0);
    });