@tuanphan I am having a similar problem. Using Squarespace 7.0 Brine template with index navigation and fixed header that becomes smaller using jQuery.
problem: Every time I click on index navigation to scroll down to a page, it off by the size of head due to its fixed nature. Please let me know if something can be done.
link: https://rose-octahedron-r9ek.squarespace.com/respire
password: 12345