I am having a hard time finding anything related to the mobile menu. My problem is that in the Magento 2 default (blank) theme it's not possible to toggle categories collapsible in the mobile menu. This means that if you have a lot of subcategories (and categories therein) you will have a very large list of items on the mobile menu.
What I'd like is to have all the menu's collapsible and be collapsed by default. This would mean that in the image below "Tops" and "Bottoms" would be collapsible and collapsed by default. I am currently trying to extend the menu.js, but there are no examples on how to do this. Any help is appreciated :)