{% assign topnav = site.data[page.topnav] %}
{% assign topnav_dropdowns = site.data[page.topnav].topnav_dropdowns %}
{% for folder in topnav.topnav %}
{% comment %}
Notice:
Liquid engine is very poor in working with boolean logic.
Also, somehow, it can not check equality of folder.url == '/' and page.url=='/' - something buggy here.
That's why below lines of li class are such ugly.
{% endcomment %}