Deprecate fixedHeader in favour of "position: sticky" based solution

When you use FixedHeader in Firefox you will see an error in the console:
"This site appears to use a scroll-linked positioning effect. This may not work well with asynchronous panning; see https://developer.mozilla.org/docs/Mozilla/Performance/ScrollLinkedEffects for further details and to join the discussion on related tools and features!"

I intend to switch my application over from using FixedHeader to using position: sticky, as recommended in the above article. Once I have done so I will post my findings here as I suspect it would be better for FixedHeader to either be rewritten making use of position: sticky, or for it to be entirely deprecated and documentation written to help people write their own fixed header CSS.

Review the Firefox warning and the linked MDN scroll-linked-effects guidance, then inspect how FixedHeader currently provides fixed positioning. Determine whether position: sticky can replace the existing behavior across the relevant use cases; done means a clear decision to rewrite or deprecate FixedHeader, with documentation describing the chosen path.