Feature summary (what you would like to be able to do and where):
Actually, the sticky header is available only in some namespaces (see this section for details).

This has been reported here by a French editor

It is curious that the Project namespace is not included.
In my opinion, the sticky header should be available on the entire website.

Benefits (why should this be implemented?):
The sticky header should be helpful for editors, especially for senior editors, in more namespaces, for exemple in the namespace Project, to quickly edit the page or access the user menu, and to prevent the scroll of the page.