WPF ScrollViewer and Panning

I have a Window with a ScrollViewer and inside the ScrollViewer there is a Rectangle. Now I added code to drag the Rectangle which works fine. But I have no idea how to show the Scrollbars when the Rectangle is moved outside the view. I thought this will happen automatically which is not the case?

Here is my XAML:

<Window x:Class="WpfApp4.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp4"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">

    <ScrollViewer Name="_scrollViewer" CanContentScroll="True" 
                  HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Auto">
        <Rectangle Name="_myRect" Width="100" Height="100" Fill="Blue"/>
    </ScrollViewer>
</Window>

And the code behind:

public partial class MainWindow : Window
{
    private Point _origin;
    private Point _start;
    private ScaleTransform _scaleTransform = new ScaleTransform();
    private TranslateTransform _translateTransform = new TranslateTransform();

    public MainWindow()
    {
        InitializeComponent();

        var group = new TransformGroup();
        group.Children.Add(_scaleTransform);
        group.Children.Add(_translateTransform);  
        _myRect.RenderTransform = group;

        // Hook up events
        _myRect.MouseLeftButtonDown += _myRect_MouseLeftButtonDown;
        _myRect.MouseLeftButtonUp += _myRect_MouseLeftButtonUp;
        _myRect.MouseMove += _myRect_MouseMove;
    }

    private void _myRect_MouseMove(object sender, MouseEventArgs e)
    {
        if (_myRect.IsMouseCaptured)
        {
            Vector v = _start - e.GetPosition(this);
            _translateTransform.X = _origin.X - v.X;
            _translateTransform.Y = _origin.Y - v.Y;
        }
    }

    private void _myRect_MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
    {
        _myRect.ReleaseMouseCapture();
        this.Cursor = Cursors.Arrow;
    }

    private void _myRect_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
    {
        _start = e.GetPosition(this);
        _origin = new Point(_translateTransform.X, _translateTransform.Y);
        Cursor = Cursors.Hand;
        _myRect.CaptureMouse();
    }
}

[UPDATED]: So based on the input I got I change XAML and the code behind to the following - but still no scrollbars?

<Window x:Class="WpfApp4.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp4"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">

    <ScrollViewer Name="_scrollViewer" CanContentScroll="True" 
                  HorizontalScrollBarVisibility="Visible" VerticalScrollBarVisibility="Visible">
        <Canvas Name="_myCanvas">
            <Rectangle Name="_myRect" Width="100" Height="100" Fill="Blue" Canvas.Left="305" Canvas.Top="129"/>
        </Canvas>
    </ScrollViewer>
</Window>

Code Behind:

public partial class MainWindow : Window
{
    private Point _start;

    public MainWindow()
    {
        InitializeComponent();
        // Hook up events
        _myRect.MouseLeftButtonDown += _myRect_MouseLeftButtonDown;
        _myRect.MouseLeftButtonUp += _myRect_MouseLeftButtonUp;
        _myRect.MouseMove += _myRect_MouseMove;
    }

    private void _myRect_MouseMove(object sender, MouseEventArgs e)
    {
        if (_myRect.IsMouseCaptured)
        {
            var canvasRelativePosition = e.GetPosition(_myCanvas);
            Debug.WriteLine($"New Position: {canvasRelativePosition}");
            Canvas.SetTop(_myRect, canvasRelativePosition.Y - _start.Y);
            Canvas.SetLeft(_myRect, canvasRelativePosition.X - _start.X);
        }
    }

    private void _myRect_MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
    {
        _myRect.ReleaseMouseCapture();
        this.Cursor = Cursors.Arrow;
    }

    private void _myRect_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
    {
        _start = e.GetPosition(_myRect);
        Debug.WriteLine($"Start Position: {_start}");
        Cursor = Cursors.Hand;
        _myRect.CaptureMouse();
    }
}