The default scrolling functionality doesn’t provide a handy method to scroll to a certain position at the moment (this is something I’ve already planned as a feature for a future release) but if the long sub menu doesn’t have any additional sub levels, then a real scrollbar can be added to it. Then it’s not a problem to scroll the sub menu programmatically and focus any item when a user starts typing – here is a live example on one of our customers’ site (except that the menu scrolls on a letter link click instead of keyboard typing) – check the “Alle marken” sub menu:
https://www.kosmetikkaufhaus.de/
The additional functionality to scroll as you type is simple to add when a real scrollbar is used (it can be implemented as part of a support response).