<Window xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  Title="TextBox z odbiciem" Width="500" Height="200" Background="DarkGreen">
  <StackPanel Margin="40">
    <TextBox x:Name="textBox" FontSize="30"/>
    <Rectangle Height="{Binding ElementName=textBox, Path=ActualHeight}"
    Width="{Binding ElementName=textBox, Path=ActualWidth}">
      <Rectangle.Fill>
        <VisualBrush Visual="{Binding ElementName=textBox}"/>
      </Rectangle.Fill>
      <Rectangle.LayoutTransform>
        <ScaleTransform ScaleY="-0.75"/>
      </Rectangle.LayoutTransform>
      <Rectangle.OpacityMask>
        <LinearGradientBrush EndPoint="0,1">
          <GradientStop Offset="0" Color="Transparent"/>
          <GradientStop Offset="1" Color="#77000000"/>
        </LinearGradientBrush>
      </Rectangle.OpacityMask>
    </Rectangle>
  </StackPanel>
</Window>