http://www.456bereastreet.com/archive/200403/usability_and_flyout_menus/

http://uxmovement.com/navigation/why-hover-menus-do-users-more-harm-than-good/

https://www.usertesting.com/blog/2014/10/20/why-you-need-to-get-rid-of-your-multi-level-menu/