The following example shows that the scrollbar is not updated after an expansion or collapse. Only after a new layout by changing window size, the scrollbar is updated correctly.
Everything works fine, if the CONSTRAINED_RESIZE_POLICY is not used:

import java.util.ArrayList;

import javafx.application.Application;
import javafx.beans.property.ReadOnlyObjectWrapper;
import javafx.scene.Scene;
import javafx.scene.control.TreeItem;
import javafx.scene.control.TreeTableColumn;
import javafx.scene.control.TreeTableView;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

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

@Override
public void start(Stage primaryStage) {

TreeTableView<String> ttv = new TreeTableView<String>();
ttv.setShowRoot(false);

// comment this out to avoid the bug:
ttv.setColumnResizePolicy(TreeTableView.CONSTRAINED_RESIZE_POLICY);


TreeTableColumn<String, String> firstColumn = new TreeTableColumn<String, String>();
firstColumn.setCellValueFactory((item) -> (new ReadOnlyObjectWrapper<String>(item.getValue()
.getValue())));

TreeTableColumn<String, String> secondColumn = new TreeTableColumn<String, String>();
secondColumn.setCellValueFactory((item) -> (new ReadOnlyObjectWrapper<String>(item.getValue()
.getValue())));
ttv.getColumns().addAll(new ArrayList<TreeTableColumn<String, String>>() {
private static final long serialVersionUID = -6617763546462124289L;
{
add(firstColumn);
add(secondColumn);
}
});

TreeItem<String> treeRootItem = new TreeItem<String>("root");
treeRootItem.setExpanded(false);
createTreeItems(treeRootItem);
ttv.setRoot(treeRootItem);

StackPane root = new StackPane();
root.getChildren().add(ttv);

Scene scene = new Scene(root, 300, 250);
primaryStage.setScene(scene);
primaryStage.show();
}

private void createTreeItems(TreeItem<String> treeRootItem) {
for (int i = 0; i < 10; i++) {
TreeItem<String> childTreeItem = new TreeItem<String>("Child: " + i);
for (int j = 0; j < 10; j++) {
childTreeItem.getChildren().add(new TreeItem<String>("Child of Child: "
+ j));
}
treeRootItem.getChildren().add(childTreeItem);
}
}
}