(function () {
    $(window).scroll(function () {
        var mainNav = $("#mainNav");
        var bannerHeight = $("#banner").height();
        var windowScrollTop = $(this).scrollTop();

        if (bannerHeight === 0) {
            mainNav.removeClass("navbar-fixed-top");
            return;
        }

        if (!mainNav.hasClass("navbar-fixed-top") && windowScrollTop >= bannerHeight) {
            $(this).scrollTop(windowScrollTop - mainNav.height());
            mainNav.addClass("navbar-fixed-top");
        }
        if (mainNav.hasClass("navbar-fixed-top") && windowScrollTop < bannerHeight - mainNav.height() - 50) {
            mainNav.removeClass("navbar-fixed-top");
            $(this).scrollTop(windowScrollTop + mainNav.height() + 50);
        }
    });
})();