Expander as ListBoxItem doesn't trigger Selection

Question

I've got a ListBox with ListBoxItem using a DataTemplate that uses Expander as its container. The problem is that Expander appears to be eating up Click event (HeaderSite part of Expander to be exact) and I never get the SelectedItem if I click on Expander (but it works if you click on ListBoxItem itself).

Any idea on how to get Expander to play nicely with ListBox?

Here's a simplified Xaml that will reproduce the problem (no code behind needed):

Edit Code updated to bring closer to my actual template but screenshots are still from previous revision (the problem is same - this is just to clarify problem with first answer)

<Window x:Class="ListBoxSample.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <StackPanel>
        <TextBlock>
            <Run Text="Selected Item" />
            <Run Text="{Binding ElementName=ListBox, Path=SelectedItem}" />
        </TextBlock>
        <ListBox x:Name="ListBox">
            <ListBoxItem>
                <Expander Header="Expandable Stuff 1">
                    <ListBox>
                        <ListBoxItem>1.1</ListBoxItem>
                        <ListBoxItem>1.2</ListBoxItem>
                    </ListBox>
                </Expander>
            </ListBoxItem>
            <ListBoxItem>
                <Expander Header="Expandable Stuff 2">
                    <ListBox>
                        <ListBoxItem>2.1</ListBoxItem>
                        <ListBoxItem>2.2</ListBoxItem>
                    </ListBox>
                </Expander>
            </ListBoxItem>
        </ListBox>
    </StackPanel>
</Window>

Screenshots are pre-edit

Clicking on ListBoxItem resulting a SelectedItem:

Clicking on Expander resulting in no SelectedItem update (click was on Expander 1 as evident by dashed outline):

Answer 1

Well the following code seems to work with the disadvantage (or maybe advantage) that at each time only Selected item will be expnaded.

Apply the following 2 attributes to your 2 Expanders

IsHitTestVisible="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=ListBoxItem}, Path=IsSelected}"
IsExpanded="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=ListBoxItem}, Path=IsSelected}"

By binding IsHitTestVisible as well, it allows the elements contained within the Expander to be interacted with.

Resulting in:

            <ListBox x:Name="ListBox" IsSynchronizedWithCurrentItem="True">
                <ListBoxItem>
                    <Expander Header="Expandable Stuff 1" IsHitTestVisible="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=ListBoxItem}, Path=IsSelected}" IsExpanded="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=ListBoxItem}, Path=IsSelected}">
                        1
                    </Expander>
                </ListBoxItem>
                <ListBoxItem>
            <Expander Header="Expandable Stuff 2" IsHitTestVisible="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=ListBoxItem}, Path=IsSelected}" IsExpanded="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=ListBoxItem}, Path=IsSelected}">
                        2
                    </Expander>
                </ListBoxItem>
            </ListBox>

ANother solution with code behind would be as such:

            <ListBox x:Name="ListBox" IsSynchronizedWithCurrentItem="True">
                <ListBoxItem>
                    <Expander Header="Expandable Stuff 1" IsHitTestVisible="False" IsExpanded="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=ListBoxItem}, Path=IsSelected}">
                <StackPanel IsHitTestVisible="True">
                    <Label Content="1"/>
                </StackPanel>
            </Expander>
                </ListBoxItem>
                <ListBoxItem>
            <Expander Header="Expandable Stuff 2" ButtonBase.Click="Expander_Click_1" Tag="{Binding  RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=ListBoxItem}}">
                2                     
            </Expander>
                </ListBoxItem>
            </ListBox>

And code behind:

    private void Expander_Click_1(object sender, RoutedEventArgs e)
    {
        if (sender is Expander)
        {
            Expander senderExp = (Expander)sender;
            object obj = senderExp.Tag;
            if (obj is ListBoxItem)
            {
                ((ListBoxItem)obj).IsSelected = true;
            }
        }            
    }

Answer 2

Without code behind you could do this

<ListBox.ItemContainerStyle>
    <Style>
        <Style.Triggers>
            <EventTrigger RoutedEvent="Control.PreviewMouseLeftButtonDown">
                <BeginStoryboard>
                    <Storyboard Storyboard.TargetProperty="(Selector.IsSelected)">
                        <BooleanAnimationUsingKeyFrames Duration="0:0:0">
                            <DiscreteBooleanKeyFrame Value="True" />
                        </BooleanAnimationUsingKeyFrames>
                    </Storyboard>
                </BeginStoryboard>
            </EventTrigger>
        </Style.Triggers>
    </Style>
</ListBox.ItemContainerStyle>