To reproduce push the button 'Increase min width'
You will see that the title will get correct offset but there will be
no scroll bar until you collapse the root

import com.sun.javafx.runtime.VersionInfo;
import javafx.application.Application;
import javafx.beans.property.SimpleStringProperty;
import javafx.beans.value.ObservableValue;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.TreeItem;
import javafx.scene.control.TreeTableColumn;
import javafx.scene.control.TreeTableView;
import javafx.scene.layout.HBox;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;
import javafx.util.Callback;

public class TreeTableSample1 extends Application {

    TreeTableView<Person> treeTableView;
    TreeTableColumn<Person, String> firstNameCol;

    @Override
    public void start(Stage primaryStage) {

        initTreeTableView();

        HBox root = new HBox(10.0);
        root.getChildren().add(treeTableView);

        Button action = new Button("Increase min width");
        action.setOnAction(new EventHandler<ActionEvent>() {

            @Override
            public void handle(ActionEvent actionEvent) {
                System.out.println("Setting min width to 500");
                firstNameCol.setMinWidth(500);
            }
        });

        VBox vb = new VBox(10.0);
        vb.getChildren().add(action);

        root.getChildren().add(vb);

        Scene scene = new Scene(root, 400, 300);

        primaryStage.setTitle(VersionInfo.getRuntimeVersion());
        primaryStage.setScene(scene);
        primaryStage.show();
    }

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

    private void initTreeTableView() {
        treeTableView = new TreeTableView<Person>();
        treeTableView.setMinSize(200, 150);
        treeTableView.setPrefSize(200, 200);
        treeTableView.setMaxSize(200, 250);
        treeTableView.setColumnResizePolicy(TreeTableView.CONSTRAINED_RESIZE_POLICY);

        TreeItem<Person> root = new TreeItem<Person>(new Person("Anna"));
        root.setExpanded(true);
        treeTableView.setRoot(root);
        treeTableView.showRootProperty().set(true);

        firstNameCol = new TreeTableColumn<Person, String>("First name");
        firstNameCol.setCellValueFactory(new Callback<TreeTableColumn.CellDataFeatures<Person, String>, ObservableValue<String>>() {
            @Override
            public ObservableValue<String> call(TreeTableColumn.CellDataFeatures<Person, String> p) {
                return p.getValue().getValue().firstName;
            }
        });
        firstNameCol.setMinWidth(80d);

        treeTableView.getColumns().addAll(firstNameCol);

        Person p = new Person("Bob");
        treeTableView.getRoot().getChildren().add(new TreeItem<Person>(p));

        p = new Person("Cindy");
        treeTableView.getRoot().getChildren().add(new TreeItem<Person>(p));

        p = new Person("Zack");
        treeTableView.getRoot().getChildren().add(new TreeItem<Person>(p));
    }

    class Person implements Comparable{

        public final SimpleStringProperty firstName;

        public Person(String name) {
            this.firstName = new SimpleStringProperty(name);
        }

        public String toString() {
            return firstName.get();
        }

        @Override
        public int compareTo(Object o) {
            return this.firstName.get().compareTo(((Person) o).firstName.get());
        }
    }
}