Look at the attached movie.

When I do autosize, width is set so that text is truncated.

Code:
    TreeTableView<Data> treeTableView;

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

    @Override
    public void start(Stage stage) throws Exception {
        treeTableView = new TreeTableView<Data>();
        treeTableView.setMaxWidth(100);
        treeTableView.setRoot(new TreeItem<Data>(new Data("Root")));
        for (int i = 0; i < 100; i++) {
            treeTableView.getRoot().getChildren().add(new TreeItem<Data>(new Data("item " + String.valueOf(i))));
        }

        treeTableView.getRoot().setExpanded(true);

        treeTableView.getColumns().add(new TreeTableColumn<Data, String>("Data"));
        ((TreeTableColumn<Data, String>) treeTableView.getColumns().get(0)).setCellValueFactory(new Callback<CellDataFeatures<Data, String>, ObservableValue<String>>() {
            @Override
            public ObservableValue<String> call(CellDataFeatures<Data, String> p) {
                return p.getValue().getValue().property;
            }
        });

        HBox vb = new HBox(5);
        vb.getChildren().addAll(treeTableView, new Button("Some button"));

        Scene scene = new Scene(vb, 300, 300);
        stage.setScene(scene);
        stage.show();
    }

    class Data {

        public StringProperty property;

        public Data(String data) {
            property = new SimpleStringProperty(data);
        }

        @Override
        public String toString() {
            return property.get();
        }
    }

Affected tests:
ControlsAutomatedTestSuite/javafx/scene/control/test/treetable/TreeTableAsTableTest/columnAutoSize