Execute test below - the comboBox appears with two arrow buttons. The unwanted button appears at the lop left corner of the combobox. The issue occurs only if a custom TabPanSkin is installed.


package test.combobox;

import javafx.application.Application;
import javafx.geometry.Insets;
import javafx.scene.Node;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.control.ComboBox;
import javafx.scene.control.Tab;
import javafx.scene.control.TabPane;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.FlowPane;
import javafx.stage.Stage;

public class ComboTabPaneTest extends Application{
  public static void main(String[] args) {
    Application.launch(args);
  }

  @Override
  public void start(Stage stage) {
    stage.setScene(new Scene(createContent()));
    stage.setTitle(getClass().getSimpleName());
    stage.show();
  }

  private Parent createContent() {
    TabPane tabPane = new TabPane();
    tabPane.getTabs().add(new Tab("Tab", createTabContent()));
    // custom skin installs unwanted controls for some reason - Note: update package if not in text.combobox
    tabPane.setStyle("-fx-skin: 'test.combobox.ComboTabPaneTest$TabPaneSkin'");
    
    BorderPane content = new BorderPane();
    content.setCenter(tabPane);
    return content;
  }
  
  private Node createTabContent(){
    FlowPane p = new FlowPane();
    p.setPadding(new Insets(40));
    p.getChildren().add(new ComboBox<>());
    return p;
  }
  
  
  // TabPaneSkin class
  public static class TabPaneSkin extends com.sun.javafx.scene.control.skin.TabPaneSkin {
    public TabPaneSkin(TabPane tabPane){
      super(tabPane);
    }
  }
}