[Android] Fix grouped CollectionView grid row spacing regression

src/Controls/src/Core/Handlers/Items/Android/SpacingItemDecoration.cs

@@ -13,10 +13,11 @@ public class SpacingItemDecoration : RecyclerView.ItemDecoration
 
 		public int VerticalOffset { get; }
 
-		int _span = 1;
-
 		ItemsLayoutOrientation _orientation;
 
+		int _cachedLastRowCol = -1;
+		int _cachedItemCount = -1;
+
 		public SpacingItemDecoration(Context context, IItemsLayout itemsLayout)
 		{
 			// The original "SpacingItemDecoration" applied spacing based on an item's current span index.
@@ -39,7 +40,6 @@ public SpacingItemDecoration(Context context, IItemsLayout itemsLayout)
 				case GridItemsLayout gridItemsLayout:
 					horizontalOffset = gridItemsLayout.HorizontalItemSpacing / 2.0;
 					verticalOffset = gridItemsLayout.VerticalItemSpacing / 2.0;
-					_span = gridItemsLayout.Span;
 					_orientation = gridItemsLayout.Orientation;
 					break;
 				case LinearItemsLayout listItemsLayout:
@@ -84,10 +84,31 @@ public override void GetItemOffsets(ARect outRect, AView view, RecyclerView pare
 			outRect.Top = VerticalOffset;
 
 			// Remove spacing on the outer edges so spacing only appears between items.
-			// A linear layout is effectively span=1, so the same math works for both.
-			int rowCol = _span <= 1 ? position : position / _span;
-			int totalRowsCols = _span <= 1 ? itemCount : (itemCount + _span - 1) / _span;
-			int lastRowCol = totalRowsCols - 1;
+			int rowCol;
+			int lastRowCol;
+
+			if (parent.GetLayoutManager() is GridLayoutManager gridLayoutManager)
+			{
+				// Use SpanSizeLookup instead of position/spanCount so full-span items
+				// (group headers, footers, etc.) are accounted for when determining rows.
+				var spanSizeLookup = gridLayoutManager.GetSpanSizeLookup();
+				int spanCount = gridLayoutManager.SpanCount;
+				rowCol = spanSizeLookup.GetSpanGroupIndex(position, spanCount);
+
+				if (_cachedItemCount != itemCount)
+				{
+					_cachedLastRowCol = spanSizeLookup.GetSpanGroupIndex(itemCount - 1, spanCount);
+					_cachedItemCount = itemCount;
+				}
+
+				lastRowCol = _cachedLastRowCol;
+			}
+			else
+			{
+				// Linear layout: each item occupies exactly one row/column.
+				rowCol = position;
+				lastRowCol = itemCount - 1;
+			}
 
 			if (_orientation == ItemsLayoutOrientation.Vertical)
 			{

src/Controls/tests/TestCases.HostApp/Issues/Issue35700.cs

@@ -0,0 +1,91 @@
+using System.Collections.ObjectModel;
+using Microsoft.Maui.Controls.Shapes;
+
+namespace Maui.Controls.Sample.Issues;
+
+[Issue(IssueTracker.Github, 35700,
+	"Grouped CollectionView items not rendered properly on Android with GridItemsLayout",
+	PlatformAffected.Android)]
+public class Issue35700 : TestContentPage
+{
+	protected override void Init()
+	{
+		var collectionView = new CollectionView2
+		{
+			AutomationId = "TestCollectionView",
+			IsGrouped = true,
+			HorizontalOptions = LayoutOptions.Fill,
+			Margin = new Thickness(5, 30, 5, 5),
+			ItemsLayout = new GridItemsLayout(ItemsLayoutOrientation.Vertical)
+			{
+				Span = 5,
+				VerticalItemSpacing = 30,
+				HorizontalItemSpacing = 10,
+			},
+			GroupHeaderTemplate = new DataTemplate(() =>
+			{
+				var label = new Label
+				{
+					HorizontalOptions = LayoutOptions.Fill,
+					HorizontalTextAlignment = TextAlignment.Start,
+					Padding = new Thickness(10),
+					FontSize = 18,
+					TextColor = Colors.White,
+					FontAttributes = FontAttributes.Bold,
+					BackgroundColor = Colors.Gray,
+				};
+				label.SetBinding(Label.TextProperty, "Name");
+				return label;
+			}),
+			ItemTemplate = new DataTemplate(() =>
+			{
+				var label = new Label
+				{
+					HorizontalOptions = LayoutOptions.Center,
+					TextColor = Colors.White,
+					VerticalOptions = LayoutOptions.Center,
+					HorizontalTextAlignment = TextAlignment.Center,
+				};
+				label.SetBinding(Label.TextProperty, ".");
+
+				return new Border
+				{
+					StrokeShape = new RoundRectangle { CornerRadius = 10 },
+					Padding = new Thickness(5),
+					MinimumWidthRequest = 50,
+					Stroke = Colors.Transparent,
+					BackgroundColor = Colors.Gray,
+					StrokeThickness = 1,
+					HorizontalOptions = LayoutOptions.Center,
+					Content = label,
+				};
+			}),
+		};
+
+		collectionView.ItemsSource = new ObservableCollection<NumberGroup35700>
+		{
+			new NumberGroup35700("100s", new List<string>
+			{
+				"100", "200", "300", "400", "500",
+				"600", "700", "800", "900",
+			}),
+			new NumberGroup35700("1000s", new List<string>
+			{
+				"1000", "2000", "3000", "4000", "5000",
+				"6000", "7000", "8000", "9000",
+			}),
+		};
+
+		Content = collectionView;
+	}
+}
+
+public class NumberGroup35700 : ObservableCollection<string>
+{
+	public string Name { get; private set; }
+
+	public NumberGroup35700(string name, List<string> numbers) : base(numbers)
+	{
+		Name = name;
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue35700.cs

@@ -0,0 +1,20 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+public class Issue35700 : _IssuesUITest
+{
+	public Issue35700(TestDevice device) : base(device) { }
+
+	public override string Issue => "Grouped CollectionView items not rendered properly on Android with GridItemsLayout";
+
+	[Test]
+	[Category(UITestCategories.CollectionView)]
+	public void GroupedCollectionViewGridLayoutRendersCorrectly()
+	{
+		App.WaitForElement("TestCollectionView");
+		VerifyScreenshot();
+	}
+}