context-menu-functions changing

Question

Running emacs 28.1 whenever I use a mode derived from text-moe or prog-mode the value of context-menu-function is replaced. What is doing this and how can I stop it?

The default is

(context-menu-undo
 context-menu-region
 context-menu-middle-separator
 context-menu-local
 context-menu-minor)

For elisp modes I see

(t
 prog-context-menu
 elisp-context-menu)

I have run this using emacs -Q so it is not any of my code or any package. I have grepped the emacs source code and I think the variable is only alteed by add-hook e.g. in emacs-lisp.el

 (add-hook 'context-menu-functions #'elisp-context-menu 10 t)

Answer 1

The issue is that I did not understand add-hook.

If it is asked to add with the LOCAL variable set it does not just copy the global value and add to it in the buffer local value.

It creates a new local value with a t "That acts as a flag to run the hook functions of the global value as well as in the local value"