Can you open ticket in our system so we can send you screenshots and explain you more clearly.
There are two solutions of your problem:
First one is to disable the "sticky" header functionality - the header and the menu will scoll together with the whole page.
Second solutions is to implement custom scroll inside the mega menu.
Please let us know which one you prefer.
Thank you for informing us about this issue!
We have fixed it without changing the markup. The fix is included in our last version of the theme and now you can update yours. Here is a guidance how to update your theme https://www.nop-templates.com/how-to-update-a-nopcommerce-theme .
Could you please let us know if it was a problem on your side or there was some issue in the theme?