Lead Developer
It’s not so much about the grid, it’s about the sub-menu appearing above the parent item when it’s actually below it in the HTML structure.
With the above CSS, it displays it below so the parent item is still visible.
Documentation: http://docs.generatepress.com/
Adding CSS: http://docs.generatepress.com/article/adding-css/
Ongoing Development: https://generatepress.com/ongoing-development