Gtk.ToggleButton.prototype.set_mode

function set_mode(draw_indicator:Boolean):void {
    // Gjs wrapper for gtk_toggle_button_set_mode()
}

Sets whether the button is displayed as a separate indicator and label. You can call this function on a checkbutton or a radiobutton with draw_indicator = false to make the button look like a normal button

This function only affects instances of classes like Gtk.CheckButton and Gtk.RadioButton that derive from Gtk.ToggleButton, not instances of Gtk.ToggleButton itself.