WPF Marquee Text Animation

Something like this should do the trick.

You can add a Canvas to the StackPanel with 2 TextBlocks one set to position 0 and one set to the ActualWidth of the StackPanel, then when the first block of text goes offscreen the other block will come into view.

The reason I used Canvas is because Canvas is the only element that actually supports ClipToBounds="false" this allows the 2nd TextBlock to be visible even if its placed outside the bounds of the Canvas itself

We also need a IValueConverter to get the correct negative value if you want to scroll from right to left.

I also added event trigger on SizeChanged so if the window is resized the animation values will update correctly.

Code:

namespace WpfApplication9
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {

        public MainWindow()
        {
            InitializeComponent();
        }
    }

    public class NegatingConverter : IValueConverter
    {

        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            if (value is double)
            {
                return -((double)value);
            }
            return value;
        }

        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            if (value is double)
            {
                return +(double)value;
            }
            return value;
        }
    }
}

Xaml:

<Window x:Class="WpfApplication9.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:WpfApplication9"
        Title="MainWindow" Height="83" Width="222" Name="UI" Tag="Tol Level">
    <StackPanel Orientation="Horizontal" x:Name="stack">
        <StackPanel.Resources>
            <local:NegatingConverter x:Key="NegatingConverter" />
            <Storyboard x:Key="slide">
                <DoubleAnimation From="0" To="{Binding Width, ElementName=canvas, Converter={StaticResource NegatingConverter}}" Duration="00:00:10"
                      Storyboard.TargetProperty="X"
                      Storyboard.TargetName="transferCurreny"
                      RepeatBehavior="Forever"/>
            </Storyboard>
        </StackPanel.Resources>
        <StackPanel.RenderTransform>
            <TranslateTransform x:Name="transferCurreny" X="0"/>
        </StackPanel.RenderTransform>
        <StackPanel.Triggers>
            <EventTrigger RoutedEvent="StackPanel.Loaded">
                <BeginStoryboard Storyboard="{StaticResource slide}" />
            </EventTrigger>
            <EventTrigger RoutedEvent="StackPanel.SizeChanged">
                <BeginStoryboard Storyboard="{StaticResource slide}" />
            </EventTrigger>
        </StackPanel.Triggers>
        <Canvas x:Name="canvas" Width="{Binding ActualWidth, ElementName=stack}">
            <TextBlock Text="StackOverflow" FontSize="25"  x:Name="txtKron" Canvas.Left="0"/>
            <TextBlock Text="{Binding Text, ElementName=txtKron}" FontSize="25" Canvas.Left="{Binding Width, ElementName=canvas}"/>
        </Canvas>
    </StackPanel>
</Window>

Result:

Edit: Left to Right

 <StackPanel Orientation="Horizontal" x:Name="stack">
        <StackPanel.Resources>
            <local:NegatingConverter x:Key="NegatingConverter" />
            <Storyboard x:Key="slide">
                <DoubleAnimation From="0" To="{Binding Width, ElementName=canvas}" Duration="00:00:10"
                      Storyboard.TargetProperty="X"
                      Storyboard.TargetName="transferCurreny"
                      RepeatBehavior="Forever"/>
            </Storyboard>
        </StackPanel.Resources>
        <StackPanel.RenderTransform>
            <TranslateTransform x:Name="transferCurreny" X="0"/>
        </StackPanel.RenderTransform>
        <StackPanel.Triggers>
            <EventTrigger RoutedEvent="StackPanel.Loaded">
                <BeginStoryboard Storyboard="{StaticResource slide}" />
            </EventTrigger>
            <EventTrigger RoutedEvent="StackPanel.SizeChanged">
                <BeginStoryboard Storyboard="{StaticResource slide}" />
            </EventTrigger>
        </StackPanel.Triggers>
        <Canvas x:Name="canvas" Width="{Binding ActualWidth, ElementName=stack}">
            <TextBlock Text="StackOverflow" FontSize="25"  x:Name="txtKron" Canvas.Left="0"/>
            <TextBlock Text="{Binding Text, ElementName=txtKron}" FontSize="25" Canvas.Left="{Binding Width, ElementName=canvas, Converter={StaticResource NegatingConverter}}"/>
        </Canvas>
    </StackPanel>