Why does the ExtentWidth stay at 10.003? - wpf

Question

So I have a ListBox that has a DataTemplate which has a Grid which has a RichTextBox.
For some reason when you type into the RichTextBox it puts each character on a separate line. Digging into this, I find out that the ExtentWidth is equal to 10.003. Why? I have no idea. I was hoping someone could explain to me why and give a nice solution to make it stop doing this.
I did notice that if you set a width on the grid's column, it fixes it, but I don't want a static width on my grid's column.
Below is an example of the problem. I am using .Net 4 and VS 2010.
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:WpfApplication1"
Title="MainWindow" Height="350" Width="525">
<ListBox
DockPanel.Dock="Top"
x:Name="TestListBox">
<ListBox.ItemTemplate>
<DataTemplate DataType="{x:Type local:Test}">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="50" />
<ColumnDefinition />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition />
<RowDefinition />
</Grid.RowDefinitions>
<TextBox Grid.Column="0" Grid.Row="0" Text="{Binding Name}" />
<TextBlock Grid.Column="1" Grid.Row="0" Text="Test" />
<RichTextBox
Grid.Column="1" Grid.Row="1"/>
</Grid>
</DataTemplate>
</ListBox.ItemTemplate>
<local:Test Name="Test1" />
<local:Test Name="Test2" />
</ListBox>
using System.Windows;
namespace WpfApplication1
{
/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
}
}
public class Test
{
public string Name { get; set; }
public Test()
{
}
}
}

Answer 1

So I figured out a workaround. If you trade out the ListBox for a DataGrid and use DataGridTemplateColumns with the Width of the TemplateColumn set to "*" then it seems to work.
<DataGrid
x:Name="TestDataGrid"
CanUserReorderColumns="False"
CanUserResizeColumns="False"
>
<DataGrid.Columns >
<DataGridTemplateColumn Width="*">
<DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<RichTextBox />
</DataTemplate>
</DataGridTemplateColumn.CellTemplate>
</DataGridTemplateColumn>
</DataGrid.Columns>
<local:Test Name="Test1" />
<local:Test Name="Test2" />
</DataGrid>
Note I also set the CanUserResizeColumns to False because if you don't and they resize them, the ExtentWidth will snap back to 10.