code: @Override
      protected void configureList() {
          list.setFont(comboBox.getFont());
          list.setBorder(null);
          list.setCellRenderer(comboBox.getRenderer());
          list.setFocusable(false);
          list.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
          int selectedIndex = comboBox.getSelectedIndex();
  
          if (selectedIndex == -1) {
              list.clearSelection();
          } else {
              list.setSelectedIndex(selectedIndex);
              list.ensureIndexIsVisible(selectedIndex);
          }
  
          installListListeners();
      }
docs: Configures the list which is used to hold the combo box items in the
  popup. This method is called when the UI class is created.