Hi All,

I want to display 1M (may be more) items in TreeView and build tree in background. I am noticed that ScrollBar of TreeView hangs during building of tree. Can you fix it or provide workaround?

Steps for reproduce:

1) Run test application.
2) Click 'Build Tree in Background' button
3) Try to scroll tree use vertical scroll bar.

After loading ~ 300 000 of items impossible to use scroll bar (it just freeze). After loading of 1M items it become scrollable again.

Expected behavior: TreeView should be `scrollable` during building tree in background.

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package com.company;

import javafx.application.Application;
import javafx.application.Platform;
import javafx.concurrent.Service;
import javafx.concurrent.Task;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.control.TreeItem;
import javafx.scene.control.TreeView;
import javafx.scene.layout.HBox;
import javafx.scene.layout.Priority;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

import java.util.ArrayList;
import java.util.List;
import java.util.UUID;

public class Main extends Application {

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

    @Override
    public void start(Stage stage) throws Exception {
        VBox vBox = new VBox();

        TreeView<String> treeView = new TreeView<>();
        treeView.setShowRoot(false);
        treeView.setRoot(new TreeItem<>());
        VBox.setVgrow(treeView, Priority.ALWAYS);
        vBox.getChildren().addAll(treeView);

        HBox hBox = new HBox();
        vBox.getChildren().add(hBox);

        Label label = new Label();
        HBox.setHgrow(label, Priority.ALWAYS);
        hBox.getChildren().add(label);

        Button button = new Button("Build Tree in Background");
        button.setOnAction(event -> {
            treeView.getRoot().getChildren().clear();

            TreeBuilder treeBuilder = new TreeBuilder(treeView);

            label.textProperty().bind(treeBuilder.messageProperty());

            treeBuilder.start();
        });
        hBox.getChildren().add(button);


        Scene scene = new Scene(vBox, 1024, 768);

        stage.setScene(scene);
        stage.show();
    }

    private class TreeBuilder extends Service {

        private TreeView<String> treeView;

        public TreeBuilder(TreeView<String> treeView) {
            this.treeView = treeView;
        }

        @Override
        protected Task createTask() {
            return new Task() {
                @Override
                protected Object call() throws Exception {
                    List<String> chunk = new ArrayList<>();
                    for (int i = 0; i < 1_000_000; i++) {
                        chunk.add(UUID.randomUUID().toString());

                        if (chunk.size() % 10_000 == 0) {
                            List<TreeItem<String>> treeItems = new ArrayList<>();
                            for (String s : chunk) {
                                treeItems.add(new TreeItem<>(s));
                            }

                            Platform.runLater(() -> treeView.getRoot().getChildren().addAll(treeItems));

                            chunk.clear();

                            updateMessage(String.valueOf(i));

                            Thread.sleep(100); // The same problem without sleep too
                        }
                    }

                    return null;
                }
            };
        }
    }

}