Size-to-content-layout problem in WPF

<Window x:Class="SO1.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Window1" SizeToContent="Width" FontSize="20">
    <DockPanel>
        <Button DockPanel.Dock="Top">A rather long text</Button>
        <TextBlock TextWrapping="Wrap">Short text</TextBlock>
    </DockPanel>
</Window>

<Button x:Name="btn" DockPanel.Dock="Top">Short text</Button>
<TextBlock TextWrapping="Wrap"
  MaxWidth="{Binding ElementName=btn,Path=ActualWidth}">A rather long text</TextBlock>