To reproduce run following code
Expected: red square in the center of third part and symmetric divider's positions


import javafx.application.Application;
import javafx.builders.SplitPaneBuilder;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.control.SplitPane;
import javafx.scene.layout.VBox;
import javafx.scene.paint.Color;
import javafx.scene.shape.Rectangle;
import javafx.stage.Stage;

public class SplitPaneLayout extends Application {

    public static void main(String[] args) {
        launch(args);
    }

    private Parent getContent() {

        VBox list = new VBox();
        list.setManaged(false);
        SplitPane pane = SplitPaneBuilder.create().items(new Rectangle(40, 40, Color.WHITESMOKE), new Rectangle(40, 40, Color.BLUE), new Rectangle(40, 40, Color.RED)).prefWidth(150).prefHeight(150).build();
        pane.setDividerPositions(0.33, 0.67);
        pane.setFocusTraversable(false);
// pane.setStyle("-fx-border-color:green; -fx-padding: 10 5 30 15;");
        pane.setMaxSize(150,150);
        list.getChildren().addAll(pane);
        return list;
    }

    public void start(Stage stage) {
        stage.setX(100);
        stage.setY(100);
        stage.setWidth(300);
        stage.setHeight(300);
        Scene scene = new Scene(getContent());
        stage.setScene(scene);
        stage.setVisible(true);
    }
}