Category Menu drop down area Disappears when scrolling up

How to keep the Main Category menu and drop down menus visible while scrolling down (not to stay sticky but to work like the demo so the entire category menu stays dropped down and scrolls with the window). Currently as I scroll down the category menu and its drop down area disappears too soon when scrolling down so cannot access the entire drop down menu. How to keep the menu permanently dropped down and not to disappear before it has scrolled from view….