为什么WPF的TextBlock的滚动条不成功？下面这代码有什么问题吗？

XML code

<Window x:Class="test.MainWindow"        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"        Title="MainWindow" Height="350" Width="525" FontSize="50">    <Grid>        <TextBlock TextWrapping="Wrap"                   ScrollViewer.VerticalScrollBarVisibility="Auto">        abcdefghijklmnopaaaa aaabbbbbbb ccccccccc cdddddddeeeeeeeeefffffffffggggggggghhhh        </TextBlock>    </Grid></Window>