✅ TreeViewItem Expanded event in MVVM

    <DockPanel>
        <TreeView ItemsSource="{Binding Items}">
            <TreeView.ItemTemplate>
                <HierarchicalDataTemplate 
                    DataType="{x:Type models:NavigationPaneItem}" 
                    ItemsSource="{Binding Children}"
                >
                    <TextBlock Text="{Binding Name}"/>
                </HierarchicalDataTemplate>
            </TreeView.ItemTemplate>
        </TreeView>