$(window).on('scroll', function(){
        (!window.requestAnimationFrame) ? fixGallery() : window.requestAnimationFrame(fixGallery);
    });


    function fixGallery() {
        var offsetTop = $('.cd-main-content').offset().top,
            scrollTop = $(window).scrollTop();
        ( scrollTop >= offsetTop ) ? $('.cd-main-content').addClass('is-fixed') : $('.cd-main-content').removeClass('is-fixed');
    }