How to add context menu actions?

[Ambrevar]

This might be more of a GTK question than a Common Lisp question, but allow me to post here, just in case. I'm trying to create a custom context menu in WebKitGTK (using cl-webkit).

A simple attempt:

; Helper to populate the browser "context-menu-entries" slot:
(defun make-context-menu-entries ()
    (let ((foo-action (gio:g-simple-action-new
                                         "foo"
                                         (glib:g-variant-type-new glib:+g-variant-type-maybe+))))

        (gobject:g-signal-connect
         foo-action "activate"
         (lambda (action parameter)
             (declare (ignore action parameter))
             (log:info "foo action")))

        (gio:g-simple-action-set-enabled foo-action t)

        (list :foo
                    (webkit:webkit-context-menu-item-new-from-gaction
                     foo-action
                     "Do foo"
                     (cffi:null-pointer)))))

; Initialize the web view:
; ...
(gobject:g-signal-connect (gtk-object buffer) "context-menu"

     (lambda (web-view context-menu event hit-test-result)
         (declare (ignore web-view event hit-test-result))

         (webkit:webkit-context-menu-append
            context-menu
            (getf (context-menu-entries *browser*) :foo))
         ;; Return nil to show the context menu.
         nil))

It works and shows "Do foo" in the context menu; sadly, it's greyed out. It seems that g-simple-action-set-enabled does not do what I think it does.

Any clue what I'm missing?