> but now noticed that after typing 'C-h m' it fails with:
>
> Debugger entered--Lisp error: (args-out-of-range 1786)
>   mouse-posn-property((#<window 178 on *scratch*> 1786 (0 . 0) 0) context-menu-function)
>   context-menu-map()
>   (lambda (_) (context-menu-map))(ignore)
>   where-is-internal(ignore nil t)
>   #f(compiled-function (sym) #<bytecode -0x1145d1e4eb4c2a6>)(ignore)
>   help-fns--list-local-commands()
>   describe-mode()
>   funcall-interactively(describe-mode)
>   command-execute(describe-mode)

This bug still exists in emacs-28:

emacs-28 -Q
1. context-menu-mode
2. C-h C-n M->
3. C-h m

Debugger entered--Lisp error: (args-out-of-range #<buffer *Help*> 1 37446)
  parse-partial-sexp(1 37446)
  syntax-ppss(37446)
  context-menu-region((keymap #("Context Menu" 0 12 (hide t)) (separator-undo "--") (separator-region "--")) 109)
  #f(compiled-function (fun) #<bytecode -0x1ee96643fef4b180>)(context-menu-region)
  run-hook-wrapped(#f(compiled-function (fun) #<bytecode -0x1ee96643fef4b180>) context-menu-region)
  context-menu-map()
  (lambda (_) (context-menu-map))(ignore)
  where-is-internal(ignore nil t)
  #f(compiled-function (sym) #<bytecode -0x1403be359693d56a>)(ignore)
  help-fns--list-local-commands()
  describe-mode()
  funcall-interactively(describe-mode)
  call-interactively(describe-mode nil nil)
  command-execute(describe-mode)

I don't know if this is the right fix, but it avoids
the error by explicitly switching to window-buffer
when context-menu is called by describe-mode: