And for better overview and understanding, take alook here.
This simple menu uses the same technique of my idea…
https://www.media-junkie.com/simple-megamenu?lang=de
Take also a look into the related POST.
I hope this can solve your problem.
Good luck and happy coding.
EDIT:
By the way. The Menu which you can find on the given example, was also created with the same technique, i just showed you right now.
Yes it works not perfekt (still in developement), but should work on every browser (but also not tested).