I really have to do this mega-menu, I have no choice...

The other room

I found myself on a long project where we *needed* the new site to visually match the old one, until it was the right time to redesign. They had to start moving away from Drupal 6 now, though. So, what on Earth to do about their mega-menu...
I wanted to make the D8 site have the same functionality and look from a desktop point of view, including the mega menu, but allow us to make the site more friendly to the many slow-access mobile users the site attracts. It needs to be responsive and all those things without being "different".
This is a story of my journey into implementing the mega-menu without it being utterly awful for mobile users; how we got the data in the menu, what we discovered about D8 and menu items, even how we considered bandwidth.
I am far from a frontend expert - probably more like a fancy site-builder so this will not be a talk about cutting edge js+css etc - just how we worked around the problems and implemented something that make a difference.