The TixComboBox widget can appear in many different styles. If we set the -dropdown option to true (which is the default), the listbox will only appear when the user presses the arrow button. When -dropdown is set to false, the listbox is always shown and the arrow button will disappear because it is not needed anymore.
There is also an option called -fancy. It is set to false by default. When set to true, a tick button and a cross button will appear next to the entry widget. The tick button allows you to select again the value that's already in the ComboBox. If you press the cross button, the entry widget will be cleared.