Bootstrap 4 activate.bs.scrollspy event is not firing

For some reason explained here, when you use body for the spy element the event gets activated on the window.

        $(window).on('activate.bs.scrollspy', function () {
            console.log("This event is not firing...");
        });

Demo: https://www.codeply.com/go/aN4tfy0zU0

EDIT

The target element can be obtained in the 2nd event param:

      $(window).on('activate.bs.scrollspy', function (e,obj) {
          console.log(obj.relatedTarget);
      });

@kim gave the right answer, but I also needed to hook to a scroll-end event, which does not seem to exist - so created something simple using a timeout:

// reset scroll watcher ##
clearTimeout( jQuery.data(this, 'scrollTimer'));

jQuery(window).on('activate.bs.scrollspy', function ( e,obj ) {
        
    // console.log(obj.relatedTarget);
        
    jQuery.data( this, 'scrollTimer', setTimeout(function() {
            
        // console.log("Didn't scroll in 1/4 sec..");

        // no scroll items are marked as active ##
        if ( ! jQuery(".scrollspy-item.active")[0]){ 

            // console.log("Can't fund any active scrollspy items..");

            // add class to parent again ##
            jQuery(".list-group-item.current").addClass( 'active' );

        }

    }, 250 ) );

});

In this case, I'm checking if any "scrollspy-items" are marked as active, or not and then adding the active class to a parent item, as we use the scrollspy in place in a sidebar menu.