Make sub items in navigation block collapsable

[tomxygen]

What problem does this address?

Currently, when adding sub items in the navigation block, they're collapsed on desktop and can be revealed by hovering over the parent item.
On mobile however, they're always shown.

I am building a WooCommerce store and I'd like to place all the product categories as sub-items of the Catalog parent item.
On mobile this is the result:

What is your proposed solution?

Sub-items should be collapsable.