Thanks for the sample URL, it helped a lot!
The problem is that you don’t have the “sm” class set to your menu in the source but you add it dynamically on DOMContentLoaded via this line:
jQuery("#menu-main-menu").addClass('sm');
in the following file:
You will need to add the class in the source if you would like to avoid the flash. Since you are using WordPress, you could add the class like this in your theme:
<?php wp_nav_menu( array( 'menu' => '...', 'menu_id' => 'menu-main-menu', 'menu_class' => 'sm' ... ) ); ?>
Please let me know if you still have any issues.
