I found a problem when navigation as header and sticky navigation are enabled together. If you have a lot of sub-items in your menu, then you cannot scroll to the bottom. When sticky navigation is set to off, then the problem disappears and I can scrolling to the bottom of menu no matter how many sub items there are.
Maybe this video can explain the problem better than me – https://filedn.com/lD3ffmBB0zs4tNK7cLXFG3Q/Screencast%202019-05-02.mp4
There isn’t much you can do about this, for really long menus on mobile we would suggest using the Slideout Navigation instead. For Desktop, i haven’t ever seen a site where the menu would extend that far, normally this type of menu would be converted to a mega menu so its displayed in rows.