I have a TreeView with long TreeItem labels in a Popup.
When I scroll the tree horizontally very quickly, the Popup randomly moves a random amount to the left.
This does not occur if the labels are short and popup is narrow, only when labels are long and popup is wide.

The source below reproduces the bug.
public class PopupPosition extends Application {

    @Override
    public void start(final Stage primaryStage) throws Exception {
        Popup popup = new Popup();

        TreeView<String> treeView = new TreeView<>();
        TreeItem<String> rootItem = new TreeItem<>("root");
        rootItem.setExpanded(true);
        addChildren(rootItem);

        treeView.setRoot(rootItem);
        treeView.setPrefWidth(400);
        treeView.setPrefHeight(300);

        popup.getContent().add(treeView);
        popup.setAutoHide(true);

        popup.xProperty().addListener(
                        (observable, oldValue, newValue) -> System.out.println("Position x: oldValue=" + oldValue
                                + ", newValue=" + newValue));

        Button button = new Button("Show popup");
        button.setOnAction(new EventHandler<ActionEvent>() {

            @Override
            public void handle(final ActionEvent event) {
                popup.show(primaryStage);
                popup.setX(primaryStage.getX());
                popup.setY(primaryStage.getY());
            }

        });

        StackPane root = new StackPane();
        root.getChildren().add(button);
        primaryStage.setScene(new Scene(root, 300, 250));
        primaryStage.show();
    }

    private void addChildren(final TreeItem<String> root) {
        TreeItem<String> item = root;
        for (int i = 0; i < 30; i++) {
            TreeItem<String> child = new TreeItem<>(
                    "this is a very very very very very very very very very very very very"
                            + " very very very very very very very very very very very very very"
                            + " very very very very very very very very very very very very very"
                            + " very very very very very very very very very very very very very"
                            + " very very very long child " + i);
            child.setExpanded(true);
            item.getChildren().add(child);

            item = child;
        }
    }

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

}