Expander Width Problem

[Kevin Marois]

I'm trying to create a ListBox, with it's ItemTemplate being an expander, which itself has a list in it. Here's a pic of what I'm trying to accomplish. [Pic](https://1drv.ms/u/s!AlkRTpT49yCMmgxTy4tnD-TMpXa1?e=nEvqc0) I added colors around each item to highlight what's happening. Here's the XAML

<ListBox Grid.Row="2"
Grid.Column="1"
x:Name="outerList"
Margin="2,2,2,2"
Background="Transparent"
ItemsSource="{Binding RecentItemSections}"
SelectedItem="{Binding SelectedRecentItemSection}"
HorizontalAlignment="Stretch"
BorderBrush="Yellow"
BorderThickness="2">

<ListBox.ItemTemplate>

    <DataTemplate>

        <Expander IsExpanded="{Binding IsSectionExpanded}"
                    x:Name="expander"
                    HorizontalAlignment="Stretch"
                    BorderBrush="Red"
                    BorderThickness="2">

            <Expander.Header>
                <TextBlock Text="{Binding SectionName}"
                            Foreground="White" 
                            Width="{Binding RelativeSource={RelativeSource Mode=FindAncestor,
                                AncestorType={x:Type ListBoxItem}}, Path=ActualWidth}"/>
            </Expander.Header>
                        
            <ListBox ItemsSource="{Binding RecentItems}"
                        BorderBrush="Green"
                        BorderThickness="5"
                        Margin="2,0,20,0">

                <ListBox.ItemTemplate>
                    <DataTemplate>

                        <Border BorderBrush="MediumAquamarine"
                                BorderThickness="2">

                            <StackPanel Orientation="Vertical"
                                    HorizontalAlignment="Stretch">

                                <TextBlock Text="{Binding ItemName}"
                                        FontSize="12"
                                        Margin="2"/>

                                <TextBlock Text="{Binding ItemLocation}"
                                        FontSize="10"
                                        Margin="2,0,2,2"/>

                            </StackPanel>

                        </Border>

[Lost User]

You should know what the maximum header width will be; set the .Width accordingly. (The framework doesn't know beforehand).

"Before entering on an understanding, I have meditated for a long time, and have foreseen what might happen. It is not genius which reveals to me suddenly, secretly, what I have to say or to do in a circumstance unexpected by other people; it is reflection, it is meditation." - Napoleon I