Fix CollectionView Header/Footer Toggle Issues on Windows and Android

[Copilot]

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Description

Fixes header and footer toggle issues in CollectionView on Windows and Android platforms where:

• Windows: Header/Footer content was not re-added after being set to null and then enabled again
• Android: Header/Footer was not properly removed at runtime when set to null

Changes Made

Windows (StructuredItemsViewHandler.Windows.cs)

1. Clear template when removing header/footer: Added explicit clearing of HeaderTemplate/FooterTemplate when header/footer is set to null. This ensures the native control properly releases the previous template reference.

2. Prevent duplicate logical child additions: Added a check to only add the view as a logical child if it's not already a child of the element. This prevents issues when the same View instance is toggled multiple times.

// Before
case null:
    ListViewBase.Header = null;
    break;

// After
case null:
    ListViewBase.HeaderTemplate = null;
    ListViewBase.Header = null;
    break;

Android (StructuredItemsViewAdapter.cs)

Replaced the generic NotifyDataSetChanged() with specific item notifications for better RecyclerView management:

• Header removed: NotifyItemRemoved(0) - Properly animates and removes the header item
• Header added: NotifyItemInserted(0) - Properly animates and inserts the header item
• Footer removed/added: Similar approach with correct position calculation

// Before
UpdateHasHeader();
NotifyDataSetChanged();

// After
var hadHeader = ItemsSource.HasHeader;
UpdateHasHeader();

if (hadHeader && !ItemsSource.HasHeader)
    NotifyItemRemoved(0);
else if (!hadHeader && ItemsSource.HasHeader)
    NotifyItemInserted(0);
else
    NotifyItemChanged(0);

Testing

Added comprehensive UI test CollectionViewHeaderFooterToggle that validates:

• Header can be removed and re-added
• Footer can be removed and re-added
• Multiple toggle cycles work correctly for both header and footer

Platforms Affected

• Windows ✅
• Android ✅

Related Issues

Fixes issue reported in: [Windows/Android] Header/Footer re-add and removal issues in CollectionView

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>[Windows/Android] Header/Footer re-add and removal issues in CollectionView</issue_title>
<issue_description>### Description

When the header/footer are added in CollectionView, the following issues were observed on Windows and Android platforms:

Windows: When toggling the Header/Footer button to the null state and then enabling it again, the header or footer content is not added back

Android: The header and footer are not removed at runtime when set to null.

Actual Behavior:
Windows - the header or footer content is not added back.
Android retains the old header/footer even after setting them to null.

Expected Behavior:
On Windows: Header/Footer should be re-added.
On Android: Header/Footer should be removed properly when set to null.

Steps to Reproduce

public partial class MainPage : ContentPage
{
    private CollectionView _collectionView;
    private Button _headerButton;
    private Button _footerButton;

	public MainPage()
	{
        //InitializeComponent();
		var grid = new Grid
        {
            Margin = new Thickness(20),
            RowDefinitions =
            {
                new RowDefinition { Height = GridLength.Auto },
                new RowDefinition { Height = GridLength.Auto },
                new RowDefinition { Height = GridLength.Star }
            }
        };



        // Header Label
        var headerLabel = new Label
		{
			Text = "Test for CollectionView empty view positioning",
			AutomationId = "HeaderLabel",
        };
        Grid.SetRow(headerLabel, 0);
        grid.Children.Add(headerLabel);

        // Control buttons
        var buttonGrid = new Grid
        {
            ColumnDefinitions =
            {
                new ColumnDefinition { Width = GridLength.Star },
                new ColumnDefinition { Width = GridLength.Star }
            },
            Margin = new Thickness(0, 10)
        };

        var toggleHeaderButton = new Button
        {
            AutomationId = "ToggleHeaderButton",
            Text = "Remove Header",
        };
        toggleHeaderButton.Clicked += OnToggleHeaderClicked;
        Grid.SetColumn(toggleHeaderButton, 0);
        buttonGrid.Children.Add(toggleHeaderButton);

        var toggleFooterButton = new Button
        {
            AutomationId = "ToggleFooterButton", 
            Text = "Remove Footer",
        };
        toggleFooterButton.Clicked += OnToggleFooterClicked;
        Grid.SetColumn(toggleFooterButton, 1);
        buttonGrid.Children.Add(toggleFooterButton);

        Grid.SetRow(buttonGrid, 1);
        grid.Children.Add(buttonGrid);

        // CollectionView
        _collectionView = new CollectionView
        {
			AutomationId = "CollectionView",
            ItemsSource = Array.Empty<string>(), // empty array to trigger EmptyView
            ItemTemplate = new DataTemplate(() =>
            {
                return new Label
                {
                    TextColor = Colors.Black,
                    FontSize = 16,
                    // This binding works with string items
                    BindingContext = "{Binding .}" 
                };
            })
        };

        // EmptyView
        _collectionView.EmptyView = new Label
        {
            AutomationId = "EmptyViewLabel",
            BackgroundColor = Color.FromArgb("#FFE40606"),
            Text = "EmptyView: This should show when no data.",
        };

        // Initial header
        _headerButton = new Button
        {
            AutomationId = "CollectionViewHeader",
            BackgroundColor = Colors.LightBlue,
            Text = "Header: Click me to verify EmptyView position",
        };
        _collectionView.Header = _headerButton;

        // Initial footer (null)
        _footerButton = new Button
        {
            AutomationId = "CollectionViewFooter",
            BackgroundColor = Colors.LightCoral,
            Text = "Footer: Click me to verify EmptyView position",
        };
        _collectionView.Footer = _footerButton

        Grid.SetRow(_collectionView, 2);
        grid.Children.Add(_collectionView);

        // Set ContentPage content
        Content = grid;
	}

    private void OnToggleHeaderClicked(object? sender, EventArgs e)
    {
        var button = (Button)sender!;
        
        if (_collectionView.Header != null)
        {
            _collectionView.Header = null;
            button.Text = "Add Header";
        }
        else
        {
            _collectionView.Header = _headerButton;
            button.Text = "Remove Header";
        }
    }

    private void OnToggleFooterClicked(object?  sender, EventArgs e)
    {
        var button = (Button)sender!;
        
        if (_collectionView.Footer != null)
        {
            _collectionView.Footer = null;
            button.Text = "Add Footer";
        }
        else
        {
            _collectionView.Footer = _footerButton;
         ...

</details>

Fixes dotnet/maui#31911

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