<StackPanel Width="200">
    <TextBox x:Name="tb">Hello world</TextBox>
    <Rectangle Height="40" HorizontalAlignment="Stretch">
      <Rectangle.Fill>
        <VisualBrush Visual="{Binding ElementName=tb}"  />
      </Rectangle.Fill>
      <Rectangle.RenderTransform>
        <ScaleTransform CenterY="20" ScaleY="-1" />
      </Rectangle.RenderTransform>
      <Rectangle.OpacityMask>
        <LinearGradientBrush StartPoint="0,0" EndPoint="0,1">
          <GradientStop Color="#00000000" Offset="0" />
          <GradientStop Color="#FFFFFFFF" Offset="1" />
        </LinearGradientBrush>
      </Rectangle.OpacityMask>
    </Rectangle>
  </StackPanel>