The MenuItem.ToggleSizeRequested event utilizes this delegate:

Event data is passed via the Gtk.ToggleSizeRequestedArgs parameter.

To attach a Gtk.ToggleSizeRequestedHandler to an event, add the ToggleSizeRequestedHandler instance to the event. The methods referenced by the ToggleSizeRequestedHandler instance are invoked whenever the event is raised, until the ToggleSizeRequestedHandler is removed from the event.