[iOS, macOS] Fixed CollectionView group header size changes with ItemSizingStrategy

src/Controls/src/Core/Handlers/Items2/iOS/GroupableItemsViewController2.cs

@@ -127,6 +127,8 @@ void UpdateTemplatedSupplementaryView(TemplatedCell2 cell, NSString elementKind,
 
 			var bindingContext = ItemsSource.Group(indexPath);
 
+			// Mark this templated cell as a supplementary view (header/footer)
+			cell.isSupplementaryView = true;
 			cell.isHeaderOrFooterChanged = true;
 			cell.Bind(template, bindingContext, ItemsView);
 			cell.isHeaderOrFooterChanged = false;

src/Controls/src/Core/Handlers/Items2/iOS/ItemsViewController2.cs

@@ -119,6 +119,8 @@ public override UICollectionViewCell GetCell(UICollectionView collectionView, NS
 			{
 				TemplatedCell2.ScrollDirection = ScrollDirection;
 
+				// Ensure this cell is treated as a regular item cell (not a supplementary view)
+				TemplatedCell2.isSupplementaryView = false;
 				TemplatedCell2.Bind(ItemsView.ItemTemplate, ItemsSource[indexpathAdjusted], ItemsView);
 			}
 			else if (cell is DefaultCell2 DefaultCell2)

src/Controls/src/Core/Handlers/Items2/iOS/StructuredItemsViewController2.cs

@@ -130,6 +130,7 @@ void UpdateTemplatedSupplementaryView(TemplatedCell2 cell, NSString elementKind)
 		{
 			bool isHeader = elementKind == UICollectionElementKindSectionKey.Header;
 			cell.isHeaderOrFooterChanged = true;
+			cell.isSupplementaryView = true;
 
 			if (isHeader)
 			{

src/Controls/src/Core/Handlers/Items2/iOS/TemplatedCell2.cs

@@ -40,6 +40,8 @@ public event EventHandler<LayoutAttributesChangedEventArgs2> LayoutAttributesCha
 		Size _measuredSize;
 		Size _cachedConstraints;
 
+		// Indicates the cell is being used as a supplementary view (group header/footer)
+		internal bool isSupplementaryView = false;
 		internal bool MeasureInvalidated => _measureInvalidated;
 
 		// Flags changes confined to the header/footer, preventing unnecessary recycling and revalidation of templated cells.
@@ -107,20 +109,28 @@ public override UICollectionViewLayoutAttributes PreferredLayoutAttributesFittin
 
 				if (_measureInvalidated || _cachedConstraints != constraints)
 				{
-					// Check if we should use the cached first item size for MeasureFirstItem optimization
-					var cachedSize = GetCachedFirstItemSizeFromHandler();
-					if (cachedSize != CGSize.Empty)
+					// Only use the cached first-item measurement for actual item cells (not headers/footers)
+					if (!isSupplementaryView)
 					{
-						_measuredSize = cachedSize.ToSize();
-						// Even when we have a cached measurement, we still need to call Measure
-						// to update the virtual view's internal state and bookkeeping
-						virtualView.Measure(constraints.Width, _measuredSize.Height);
+						var cachedSize = GetCachedFirstItemSizeFromHandler();
+						if (cachedSize != CGSize.Empty)
+						{
+							_measuredSize = cachedSize.ToSize();
+							// Even when we have a cached measurement, we still need to call Measure
+							// to update the virtual view's internal state and bookkeeping
+							virtualView.Measure(constraints.Width, _measuredSize.Height);
+						}
+						else
+						{
+							_measuredSize = virtualView.Measure(constraints.Width, constraints.Height);
+							// If this is the first item being measured, cache it for MeasureFirstItem strategy
+							SetCachedFirstItemSizeToHandler(_measuredSize.ToCGSize());
+						}
 					}
 					else
 					{
+						// For headers/footers, always measure directly without using or updating the first-item cache
 						_measuredSize = virtualView.Measure(constraints.Width, constraints.Height);
-						// If this is the first item being measured, cache it for MeasureFirstItem strategy
-						SetCachedFirstItemSizeToHandler(_measuredSize.ToCGSize());
 					}
 					_cachedConstraints = constraints;
 					_needsArrange = true;
@@ -194,6 +204,7 @@ public override void LayoutSubviews()
 		public override void PrepareForReuse()
 		{
 			//Unbind();
+			isSupplementaryView = false;
 			base.PrepareForReuse();
 		}
 

src/Controls/tests/TestCases.HostApp/Issues/Issue33130.xaml

@@ -0,0 +1,71 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="Maui.Controls.Sample.Issues.Issue33130"
+             xmlns:local="clr-namespace:Maui.Controls.Sample"
+             Title="Issue33130">
+    <Grid Margin="20"
+          RowDefinitions="Auto, *">
+        <StackLayout Grid.Row="0"
+                     Spacing="10"
+                     Margin="0,10">
+            <Button Text="Switch to MeasureFirstItem"
+                    Clicked="OnSwitchToMeasureFirstItem"
+                    AutomationId="SwitchStrategyButton"/>
+            <Label x:Name="StatusLabel"
+                   AutomationId="StatusLabel"
+                   Text="ItemSizingStrategy: MeasureAllItems"/>
+        </StackLayout>
+
+        <local:CollectionView2 Grid.Row="1"
+                               x:Name="TestCollectionView"
+                               ItemsSource="{Binding Animals}"
+                               IsGrouped="true"
+                               AutomationId="TestCollectionView"
+                               ItemSizingStrategy="MeasureAllItems">
+            <local:CollectionView2.ItemTemplate>
+                <DataTemplate>
+                    <Grid Padding="10">
+                        <Grid.RowDefinitions>
+                            <RowDefinition Height="Auto"/>
+                            <RowDefinition Height="Auto"/>
+                        </Grid.RowDefinitions>
+                        <Grid.ColumnDefinitions>
+                            <ColumnDefinition Width="Auto"/>
+                            <ColumnDefinition Width="*"/>
+                        </Grid.ColumnDefinitions>
+                        <Image Grid.RowSpan="2"
+                               Source="{Binding ImageUrl}"
+                               Aspect="AspectFill"
+                               WidthRequest="80"
+                               HeightRequest="80"/>
+                        <Label Grid.Column="1"
+                               Text="{Binding Name}"
+                               FontAttributes="Bold"/>
+                        <Label Grid.Row="1"
+                               Grid.Column="1"
+                               Text="{Binding Location}"
+                               FontAttributes="Italic"
+                               VerticalOptions="End"/>
+                    </Grid>
+                </DataTemplate>
+            </local:CollectionView2.ItemTemplate>
+            <local:CollectionView2.GroupHeaderTemplate>
+                <DataTemplate>
+                    <Label x:Name="GroupHeaderLabel"
+                           Text="{Binding Name}"
+                           BackgroundColor="LightGray"
+                           FontSize="20"
+                           FontAttributes="Bold"
+                           AutomationId="GroupHeader"/>
+                </DataTemplate>
+            </local:CollectionView2.GroupHeaderTemplate>
+            <local:CollectionView2.GroupFooterTemplate>
+                <DataTemplate>
+                    <Label Text="{Binding Count, StringFormat='Total animals: {0:D}'}"
+                           Margin="0,0,0,10"/>
+                </DataTemplate>
+            </local:CollectionView2.GroupFooterTemplate>
+        </local:CollectionView2>
+    </Grid>
+</ContentPage>

src/Controls/tests/TestCases.HostApp/Issues/Issue33130.xaml.cs

@@ -0,0 +1,66 @@
+using System;
+using System.Collections.ObjectModel;
+using Microsoft.Maui.Controls;
+
+namespace Maui.Controls.Sample.Issues;
+
+[Issue(IssueTracker.Github, 33130, "CollectionView group header size changes with ItemSizingStrategy", PlatformAffected.iOS | PlatformAffected.macOS)]
+public partial class Issue33130 : ContentPage
+{
+	public Issue33130()
+	{
+		InitializeComponent();
+		BindingContext = new GroupedAnimalsViewModel();
+	}
+
+	private void OnSwitchToMeasureFirstItem(object sender, EventArgs e)
+	{
+		TestCollectionView.ItemSizingStrategy = ItemSizingStrategy.MeasureFirstItem;
+		StatusLabel.Text = $"ItemSizingStrategy: {TestCollectionView.ItemSizingStrategy}";
+	}
+}
+
+public class GroupedAnimalsViewModel
+{
+	public ObservableCollection<GroupHeaderTestAnimalGroup> Animals { get; set; }
+
+	public GroupedAnimalsViewModel()
+	{
+		Animals = new ObservableCollection<GroupHeaderTestAnimalGroup>
+		{
+			new GroupHeaderTestAnimalGroup("Bears")
+			{
+				new GroupHeaderTestAnimal { Name = "Grizzly Bear", Location = "North America", ImageUrl = "bear.jpg" },
+				new GroupHeaderTestAnimal { Name = "Polar Bear", Location = "Arctic", ImageUrl = "bear.jpg" },
+			},
+			new GroupHeaderTestAnimalGroup("Monkeys")
+			{
+				new GroupHeaderTestAnimal { Name = "Baboon", Location = "Africa", ImageUrl = "monkey.jpg" },
+				new GroupHeaderTestAnimal { Name = "Capuchin Monkey", Location = "South America", ImageUrl = "monkey.jpg" },
+				new GroupHeaderTestAnimal { Name = "Spider Monkey", Location = "Central America", ImageUrl = "monkey.jpg" },
+			},
+			new GroupHeaderTestAnimalGroup("Elephants")
+			{
+				new GroupHeaderTestAnimal { Name = "African Elephant", Location = "Africa", ImageUrl = "elephant.jpg" },
+				new GroupHeaderTestAnimal { Name = "Asian Elephant", Location = "Asia", ImageUrl = "elephant.jpg" },
+			}
+		};
+	}
+}
+
+public class GroupHeaderTestAnimalGroup : ObservableCollection<GroupHeaderTestAnimal>
+{
+	public string Name { get; set; }
+
+	public GroupHeaderTestAnimalGroup(string name) : base()
+	{
+		Name = name;
+	}
+}
+
+public class GroupHeaderTestAnimal
+{
+	public string Name { get; set; }
+	public string Location { get; set; }
+	public string ImageUrl { get; set; }
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue33130.cs

@@ -0,0 +1,44 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+public class Issue33130 : _IssuesUITest
+{
+	public override string Issue => "CollectionView group header size changes with ItemSizingStrategy";
+
+	public Issue33130(TestDevice device) : base(device) { }
+	[Test]
+	[Category(UITestCategories.CollectionView)]
+	public void GroupHeaderSizeShouldNotChangeWithItemSizingStrategy()
+	{
+		// Wait for the CollectionView to load
+		App.WaitForElement("TestCollectionView");
+		App.WaitForElement("GroupHeader");
+
+		// Get the initial header size (before changing ItemSizingStrategy)
+		var headerElementBefore = App.FindElement("GroupHeader");
+		var headerRectBefore = headerElementBefore.GetRect();
+
+		Assert.That(headerRectBefore.Height, Is.GreaterThan(0), "Header should have a height before strategy change");
+
+		// Switch ItemSizingStrategy
+		App.WaitForElement("SwitchStrategyButton");
+		App.Tap("SwitchStrategyButton");
+
+		// Get the header size after changing ItemSizingStrategy
+		var headerElementAfter = App.FindElement("GroupHeader");
+		var headerRectAfter = headerElementAfter.GetRect();
+
+		Assert.That(headerRectAfter.Height, Is.GreaterThan(0), "Header should have a height after strategy change");
+
+		// The header size should remain the same (within a small tolerance for rendering differences)
+		// Allow for small rounding differences but not significant changes
+		var heightDifference = Math.Abs(headerRectBefore.Height - headerRectAfter.Height);
+
+		// Assert that the height difference is minimal (less than 5 pixels tolerance)
+		Assert.That(heightDifference, Is.EqualTo(0),
+			$"Header height should not change significantly. Before: {headerRectBefore.Height}, After: {headerRectAfter.Height}, Difference: {heightDifference}");
+	}
+}