Hi Christian,
We chose the pro theme over the X or other themes because it is marketed toward advanced needs and professional websites.
I know that we have many menu items but it is still a fairly normal amount. I would understand your concern if we had thousands of menu items or more.
I never encountered such browser behavior in any website or any browser that is caused by the number of menu items.
Please see two examples:
Our current website, which is essentially the same but using different technology, same number of menu items but no issue with IE scrolling:
Summary
More relevant example is the exact same website we built with your theme, only using the Twenty Seventeen theme. Same number of menu items also with a sticky menu and a mobile menu, all working fantastically with no issue with IE11 scrolling:
There is no way around the understanding that something in the theme is causing that problem in IE (maybe some script?).
If medium-large structure menus is something that is limited with the pro theme, I must say that the name and description of the theme are very very misleading. Please stand behind your title at: https://theme.co/pro/ “The Most Advanced Website Creator for WordPress.”
Please understand our urgency, we are launching this website very soon with many users that will access it using IE11. We would be more than happy to get paid support if it will expedite the process of fixing it.
We will welcome any creativity that will solve this issue as fast as possible.
Thanks,
Amir.