Fix inconsistent Grouped CollectionView (GridItemsLayout) rendering on Windows and iOS/macOS

src/Controls/src/Core/Handlers/Items/StructuredItemsViewHandler.Windows.cs

@@ -259,6 +259,7 @@ static WStyle GetItemContainerStyle(GridItemsLayout layout)
 			style.Setters.Add(new WSetter(FrameworkElement.MarginProperty, margin));
 			style.Setters.Add(new WSetter(Control.PaddingProperty, WinUIHelpers.CreateThickness(0)));
 			style.Setters.Add(new WSetter(Control.HorizontalContentAlignmentProperty, HorizontalAlignment.Stretch));
+			style.Setters.Add(new WSetter(Control.VerticalContentAlignmentProperty, VerticalAlignment.Stretch));
 
 			return style;
 		}

src/Controls/src/Core/Handlers/Items2/iOS/LayoutFactory2.cs

@@ -85,6 +85,35 @@ static NSCollectionLayoutBoundarySupplementaryItem[] CreateSupplementaryItems(La
 		return [];
 	}
 
+	static void ApplyHeaderFooterBoundarySpacing(NSCollectionLayoutSection section, UICollectionViewScrollDirection scrollDirection, double verticalSpacing, double horizontalSpacing, bool hasHeader, bool hasFooter, bool applySpacing)
+	{
+		if (!applySpacing)
+		{
+			return;
+		}
+
+		if (scrollDirection == UICollectionViewScrollDirection.Vertical)
+		{
+			var topInset = hasHeader ? new NFloat(verticalSpacing) : new NFloat(0);
+			var bottomInset = hasFooter ? new NFloat(verticalSpacing) : new NFloat(0);
+
+			if (topInset > 0 || bottomInset > 0)
+			{
+				section.ContentInsets = new NSDirectionalEdgeInsets(topInset, 0, bottomInset, 0);
+			}
+		}
+		else
+		{
+			var leadingInset = hasHeader ? new NFloat(horizontalSpacing) : new NFloat(0);
+			var trailingInset = hasFooter ? new NFloat(horizontalSpacing) : new NFloat(0);
+
+			if (leadingInset > 0 || trailingInset > 0)
+			{
+				section.ContentInsets = new NSDirectionalEdgeInsets(0, leadingInset, 0, trailingInset);
+			}
+		}
+	}
+
 	static UICollectionViewLayout CreateListLayout(UICollectionViewScrollDirection scrollDirection, LayoutGroupingInfo groupingInfo, LayoutHeaderFooterInfo layoutHeaderFooterInfo, LayoutSnapInfo snapInfo, NSCollectionLayoutDimension itemWidth, NSCollectionLayoutDimension itemHeight, NSCollectionLayoutDimension groupWidth, NSCollectionLayoutDimension groupHeight, double itemSpacing, Func<Thickness>? peekAreaInsetsFunc, ItemsLayout itemsLayout)
 	{
 		var layoutConfiguration = new UICollectionViewCompositionalLayoutConfiguration();
@@ -145,31 +174,14 @@ static UICollectionViewLayout CreateListLayout(UICollectionViewScrollDirection s
 			// For grouped sections with a group header/footer, add content insets to create
 			// the gap between the header/footer supplementary item and the first/last item.
 			// InterSectionSpacing (set on layoutConfiguration above) handles the gap between sections.
-			if (groupingInfo.IsGrouped && itemSpacing > 0)
-			{
-				if (scrollDirection == UICollectionViewScrollDirection.Horizontal)
-				{
-					var leadingInset = groupingInfo.HasHeader ? new NFloat(itemSpacing) : new NFloat(0);
-					var trailingInset = groupingInfo.HasFooter ? new NFloat(itemSpacing) : new NFloat(0);
-
-					if (leadingInset > 0 || trailingInset > 0)
-					{
-						section.ContentInsets = new NSDirectionalEdgeInsets(0, leadingInset, 0, trailingInset);
-					}
-				}
-				else
-				{
-					// Vertical: top inset creates gap between header and first item,
-					// bottom inset creates gap between last item and footer.
-					var topInset = groupingInfo.HasHeader ? new NFloat(itemSpacing) : new NFloat(0);
-					var bottomInset = groupingInfo.HasFooter ? new NFloat(itemSpacing) : new NFloat(0);
-
-					if (topInset > 0 || bottomInset > 0)
-					{
-						section.ContentInsets = new NSDirectionalEdgeInsets(topInset, 0, bottomInset, 0);
-					}
-				}
-			}
+			ApplyHeaderFooterBoundarySpacing(
+				section,
+				scrollDirection,
+				itemSpacing,
+				itemSpacing,
+				groupingInfo.HasHeader,
+				groupingInfo.HasFooter,
+				groupingInfo.IsGrouped && itemSpacing > 0);
 
 			// Create header and footer for group
 			section.BoundarySupplementaryItems = CreateSupplementaryItems(
@@ -192,6 +204,15 @@ static UICollectionViewLayout CreateGridLayout(UICollectionViewScrollDirection s
 		var layoutConfiguration = new UICollectionViewCompositionalLayoutConfiguration();
 		layoutConfiguration.ScrollDirection = scrollDirection;
 
+		var mainAxisSpacing = scrollDirection == UICollectionViewScrollDirection.Vertical
+			? verticalItemSpacing
+			: horizontalItemSpacing;
+
+		if (groupingInfo.IsGrouped && mainAxisSpacing > 0)
+		{
+			layoutConfiguration.InterSectionSpacing = new NFloat(mainAxisSpacing);
+		}
+
 		var layout = new CustomUICollectionViewCompositionalLayout(snapInfo, groupingInfo, headerFooterInfo, (sectionIndex, environment) =>
 		{
 			// Each item has a size
@@ -235,6 +256,14 @@ static UICollectionViewLayout CreateGridLayout(UICollectionViewScrollDirection s
 				section.InterGroupSpacing = new NFloat(horizontalItemSpacing);
 			}
 
+			ApplyHeaderFooterBoundarySpacing(
+				section,
+				scrollDirection,
+				verticalItemSpacing,
+				horizontalItemSpacing,
+				groupingInfo.HasHeader,
+				groupingInfo.HasFooter,
+				groupingInfo.IsGrouped && mainAxisSpacing > 0);
 
 			section.BoundarySupplementaryItems = CreateSupplementaryItems(
 				groupingInfo,

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

@@ -5,7 +5,7 @@ namespace Maui.Controls.Sample.Issues;
 
 [Issue(IssueTracker.Github, 35700,
 	"Grouped CollectionView items not rendered properly on Android with GridItemsLayout",
-	PlatformAffected.Android)]
+	PlatformAffected.Android | PlatformAffected.iOS | PlatformAffected.macOS | PlatformAffected.UWP)]
 public class Issue35700 : TestContentPage
 {
 	protected override void Init()