Here's a test case to reproduce it:
-------------------------------------------

#include <gtkmm/main.h>
#include <gtkmm/window.h>
#include <gtkmm/comboboxentrytext.h>
 
int main(int argc, char **argv)
{
        Gtk::Main kit(argc, argv);
 
        Gtk::ComboBoxEntryText cbet;
        cbet.append_text("Mother");
        cbet.append_text("Mary");
        cbet.append_text("Joseph");
 
        Gtk::Window window;
        window.add(cbet);
        window.show_all();
 
        Gtk::Main::run(window);
}

-------------------------------------------