This mega-menu is a page. You can add what you want on it : Blocks, layouts styles on blocks, ..
A page-type is installed with Anitya, let you create mega-menu in one click, the second one is to publish them!
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Welcome to new Anitya I'm John,
The main navigation can display rich content under a page. This content is infinite possibilities because it is a page !
This page is created from a Page-type to let choose where add the mega menu.
Documentation
