Typically, the theme navigation does not have a scrolling feature within the drop down menu. The resolution here would be to consolidate your menu into more nested items.
Essentially, you can take categories and nestle them under another top-tier item to organize your collection of products. Here's an example:
This will make your navigation more user friendly and concise. If you have any questions, please let me know.
Holly | Social Care @ Shopify - Was my reply helpful? Click Like to let me know! - Was your question answered? Mark it as an Accepted Solution - To learn more visit the Shopify Help Center or the Shopify Blog