Data Templated ListBox not stretching Horizontally - again - listbox

Question

Can't get my horizontal ListBox with 'Data Template' to stretch fully.
I researched this topic and the most popular answer is to set the ItemContainer Style like :
<ListBox.ItemContainerStyle>
<Style TargetType="ListBoxItem">
<Setter Property="HorizontalContentAlignment" Value="Stretch"/>
</Style>
</ListBox.ItemContainerStyle>
This doesn't seem to work for me.
Here is the fully XAML code. The ListBox.ItemsSource = ObservableCollection
<UserControl.Resources>
<ResourceDictionary>
<ItemsPanelTemplate x:Key="ItemsPanel">
<StackPanel Orientation="Horizontal"/>
</ItemsPanelTemplate>
<DataTemplate x:Key="ItemTemplate">
<Button Margin="-2,-2,-4,-2" >
<StackPanel>
<TextBlock Text="{Binding}"
TextWrapping="Wrap"/>
</StackPanel>
</Button>
</DataTemplate>
</ResourceDictionary>
</UserControl.Resources>
<ListBox VerticalAlignment="Top"
Height="50"
Background="Red"
ItemsPanel="{StaticResource ItemsPanel}"
ItemTemplate="{StaticResource ItemTemplate}"
ItemsSource="{Binding Headers}" >
<ListBox.ItemContainerStyle>
<Style TargetType="ListBoxItem">
<Setter Property="HorizontalContentAlignment" Value="Stretch"/>
</Style>
</ListBox.ItemContainerStyle>
</ListBox>

Answer 1

Unfortunately this is difficult to do if you really want to use data-binding with a ListBox (or in fact any ItemsControl).
The reason is that there is no standard panel that works like a StackPanel but stretches its elements in the layout direction.
I don't even know of any third-party panel that does that (which makes me sometimes wonder what it exactly is that toolkit providers are for). I have a small marker in my mind to write one myself next time I need it, because it's easy to do if you're already familiar with developing panels.
If you are not, you are probably better off with creating a Grid programmatically.
I assume you have a variable number of elements (otherwise you could just define each in xaml and wouldn't need a ListBox).
Instead of using a ListBox with data binding you could also programmatically create ToggleButtons in a Grid along with the GridColumns all set to being star-sized.
Of course you could also just change your design and have the buttons be left-justified.
I know that this is a nuisance, but that's how it is.