structural_pages/js/structural_pages_select2.js

(function ($, Drupal, once) {
    Drupal.behaviors.structuralPagesSelect2 = {
        attach: function (context, settings) {
            function initSelect2() {
                var elements = once('structural-pages-select2', 'select.select2-widget', context);
                if (elements.length) {
                    var $select = $(elements);
                    // If the select2 jQuery plugin is loaded (by another module/theme), use it.
                    // Otherwise, try chosen if it's there.
                    if (typeof $.fn.select2 !== 'undefined') {
                        $select.select2({
                            width: '100%',
                            dropdownAutoWidth: true
                        });
                    }
                    else if (typeof $.fn.chosen !== 'undefined') {
                        $select.chosen({
                            width: '100%',
                            search_contains: true
                        });
                    }
                }
            }

            // Initialize on load
            initSelect2();

            // Some admin themes replace select elements entirely. This ensures we hook in.
            $(document).on('ajaxComplete', function () {
                initSelect2();
            });
        }
    };
})(jQuery, Drupal, once);