为什么WPF进度条不能适应？

This is my original code:

这是我的原始代码：

<StackPanel Grid.Row="3" Grid.Column="0" Grid.ColumnSpan="2" Orientation="Horizontal">
        <ProgressBar Height="23" Name="searchProgressBar" Foreground="Blue" BorderBrush="#00000000" BorderThickness="1" VerticalAlignment="Top" HorizontalAlignment="Stretch"/>
        <TextBlock Text="asdf" Height="23"  Name="progressTextBlock" VerticalAlignment="Top" Foreground="Red" HorizontalAlignment="Right"/>
</StackPanel>
<StackP