I have a Windows Phone 7 ListBox that binds to a list of integers. I am using the default MVVM Light template, so there is a ViewModel class that contains data and a simple RelayCommand. Here is the ListBox:

<ListBox ItemsSource="{Binding MyData}">
    <ListBox.ItemTemplate>
        <DataTemplate>                        
            <Button Content="{Binding}">
                <i:Interaction.Triggers>
                    <i:EventTrigger EventName="Click">
                        <cmd:EventToCommand Command="{Binding ElementName=ContentGrid, Path=DataContext.TestCommand}"
                                            CommandParameter="{Binding}" />
                    </i:EventTrigger>
                </i:Interaction.Triggers>
            </Button>
        </DataTemplate>
    </ListBox.ItemTemplate>
</ListBox>

This displays a vertical list of integers inside buttons. If you click any of them, the following command code executes and shows a pop-up: new RelayCommand<int>(i => MessageBox.Show("Test" + i));

However, if I simply add the following XAML to change to a horizontal list, the databinding fails. Nothing happens when you click the button and no error messages are written to the Output window.

<ListBox.ItemsPanel>
    <ItemsPanelTemplate>
        <StackPanel Orientation="Horizontal" HorizontalAlignment="Center" />
    </ItemsPanelTemplate>
</ListBox.ItemsPanel>

I have tried some other types of binding for the EventToCommand. For example, specifying my ViewModel as a static resource. It works, but is less ideal than the example above.

Why does that ItemsPanel break the databinding?