The diff view is not a special page – it's the same namespace as in the reading and editing modes. Options like edit, history, or watch are available, so the sticky header would have its standard look.
Requested at https://www.mediawiki.org/w/index.php?title=Talk:Reading/Web/Desktop_Improvements&oldid=6175621#Sticky_header
Note for QAs: If and when the task is resolved, please check both views, when the option "show the page content below the diff" is on and off.