css - https://stackoverflow.com/questions/37963302/how-can-i-make-my-drop-down-menu-scrollable

bootstrap - https://bootsnipp.com/snippets/MaKa3

jquery - http://www.htmldrive.net/items/show/701/jquery-very-long-scroll-dropdown-menus

other-
1 https://www.codeproject.com/Questions/229885/how-to-make-scroll-bars-in-dropdown-list
2 mobile - https://forum.webflow.com/t/scrollable-nav-and-dropdown-menu-in-mobile/58290
3 bootstrap (looks promising) - https://mdbootstrap.com/support/angular/dropdown-menu-with-vertical-scrollbar/
4
5

or try something different - https://www.w3schools.com/howto/howto_css_subnav.asp