Force TextBlock to wrap in WPF ListBox

I have a WPF listbox which displays messages. It contains an avatar on the left side and the username and message stacked vertically to the right of the avatar. The layout is fine until the message text should word wrap, but instead I get a horizontal scroll bar on the listbox.

I've Googled and found solutions to similar issues, but none of them worked.

<ListBox HorizontalContentAlignment="Stretch"  ItemsSource="{Binding Path=FriendsTimeline}">
    <ListBox.ItemTemplate>
        <DataTemplate>
            <StackPanel Orientation="Horizontal">
                <Border BorderBrush="DarkBlue" BorderThickness="3" CornerRadius="2" Margin="3" >
                    <Image Height="32" Width="32"  Source="{Binding Path=User.ProfileImageUrl}"/>
                </Border>
                <StackPanel Orientation="Vertical">
                    <TextBlock Text="{Binding Path=User.UserName}"/>
                    <TextBlock Text="{Binding Path=Text}" TextWrapping="WrapWithOverflow"/> <!-- This is the textblock I'm having issues with. -->
                </StackPanel>
            </StackPanel>
        </DataTemplate>
    </ListBox.ItemTemplate>
</ListBox>

Contents of the TextBlock can be wrapped using property TextWrapping. Instead of StackPanel, use DockPanel/Grid. One more thing - set ScrollViewer.HorizontalScrollBarVisibility property to Disabled value for the ListBox.

Updated Hidden to Disabled based on comment from Matt. Thanks Matt.

The problem might not be located in the ListBox. The TextBlock won't wrap, if one of the parent controls provides enough space, so that it hasn't the need to wrap. This might be caused by a ScrollViewer control.

If you want to prevent TextBlock to grow, and you want it to just fit in the size of the listbox, you should set the width of it explicitly.

In order to change it dynamically, it means not a fix value, but you need to bind it to its proper parent element in the visual tree. You can have something like this:

<ListBox ItemsSource="{Binding MyItems}" Name="MyListBox">

  <ListBox.Resources>
    <Style TargetType="ListBoxItem">
      <Setter Property="Width" 
              Value="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=ScrollContentPresenter}, Path=ActualWidth}" />
    </Style>
  </ListBox.Resources>

  <ListBox.ItemTemplate>
    <DataTemplate>
      <TextBlock Text="{Binding Title}" TextWrapping="Wrap" />
    </DataTemplate>
  </ListBox.ItemTemplate>

</ListBox>

If it does not work, try to find the proper elements (which has to be binded to what) with the Live Visual Tree in Visual Studio.