GtkToolbar popup menu question

Hi, All,
On the page: Gtk.Toolbar::popup-context-menu, it states:

Default handler:

The default handler is called after the handlers added via g_signal_connect()

However it doesn’t say what this default handler do.

Does it show some kind of an empty context menu? Or there is some context menu items already?

I’m curious because I wonder if its possible to have some kind of toolbar setup functionality and if its already present at GTK level and I just need to override it.

Also, what would you recommend to do?
Use this signal, display the context menu and have a menu item which says “Customize…”? Or don’t bother with the context menu and just directly display customization dialog? Or maybe just connect to the left mouse button click and display the customization dialog in response to that?

What would be most in-par with the GTK guidelines?

Or maybe GTK4 introduces this functionality already?

On Windows I can directly handle left mouse click and display my customization dialog.
On OSX thre is a special flag that allows default customization, but I’m not sure if I can use my own dialog.

So what would be the best option out of those 3 above for GTK+?

Thank you.