A clean way in WinUI 3 to show group headers visually in a GridView with a flat ItemsSource while keeping selection indexing clean is not directly built into the control. However, a recommended pattern involves these core ideas:

• Use your flat ItemsSource but differentiate header items by a property or type.

• Use a DataTemplateSelector to render header items visually distinct with non-selectable UI and partial span styles.

• Make header items non-focusable and non-clickable by setting IsHitTestVisible=false.

• Intercept selection and indexing logic events (like SelectionChanged or ItemClick) to filter out header items. You maintain a mapping from the GridView index to your underlying data index by skipping the header positions internally.

• You can maintain a separate lookup (dictionary or list) in your ViewModel that maps displayed indices ignoring headers to true data indices, essentially creating a filtered index view. Use this mapping for all selection and manipulation logic so SelectedIndex maps correctly to your pure data items.

This approach avoids nested lists and CollectionViewSource, using a flat collection but requires manual mapping in code-behind or ViewModel when handling selection/indexing.

Additional tips:

• Headers can be made visually distinct by using variable sized panels such as VariableSizedWrapGrid in the header DataTemplate to span the full width.

• Using ContainerContentChanging to disable hit testing on headers is good but just stops interaction; index mapping still needs a manual approach.

• You could override the GridView's SelectedIndex property indirectly by wrapping or intercepting selection events and converting them between flat and displayed indices.