Edit: possibly solved? it appears as though Shopify is now natively arranging my level-2 menu items vertically after more items are added. I'm still curious how to achieve below if I have less fewer than 3 items...if anyone cares to comment. Otherwise problem solved at the moment.
I'm using debut. I have 2 levels in my navigation. First is "Shop" then "Clothing" "Shoes" and "Accessories". I would like these three categories to be listed vertically rather than horizontally.
What the dropdown looks like now when you click "Shop" :
Clothing Shoes Accessories
What I want the dropdown to look when you click "Shop":