Megamenu not scroll if sticky menu

Hi, I am using the MegaMenu with MegaMenu Storefront plugin together, and using the plugin a storefront site with Bookshop child theme (offically storefront theme)

The megamenu contains the categories, will be more then 100 with subcategories. On mobile menu it is perfect with or without sticky header, but on Desktop the megamenu is fixed, not scrolling if the header is also sticky header. I have to switch off sticky header to let the menu scrolled

Is there any quick fix for this?

The site is now under maintenance mode

The page I need help with: [log in to see the link]