There is a known issue with a long mobile menu and the sticky nav. Its not an issue without the sticky nav as the whole menu is static.
The problem with the background scroll is felt by many – its not a GP specific thing but a mobile OS thing, particularly iOS. And every time i find a fix for it it seems Apple fix it to not work again.
If you’re having issues with the Off Canvas then thats a different issue. And it may just be a case of adding some additional space below the last menu item so it doesn’t get caught up behind the devices UI. Happy to look at this if you want. Let me know.