Place this code in the Page Header Code Injection area of the page with your Secondar Nav. Or in the Site Header Code Injection area if you’re using this plugin site-wide. 

<script>
  (function() {
    window.addEventListener('secondaryNavLoaded', showPage);
    window.addEventListener('load', function(){
      setTimeout(function(){showPage()}, 300)
    });

    function showPage() {
      document.body.classList.add('wm-secondary-nav-css-loaded');
    }
  }());
</script>
<style>
  body{
    opacity:0;
    transition: opacity .3s ease;
  }
  body.wm-secondary-nav-css-loaded{
    opacity:1;
  }
</style>
Previous

Can I self host this plugin?
Next

Prevent Nav Flicker (with data-position="section")