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

[NanthiniMahalingam]

Root cause

• When the ItemSizingStrategy is set to measure the first item, the collection view’s grouped item header or footer template size is taken from the first item’s size instead of the actual size of the grouped item header or footer template.

Description of changes

• I have ignored the first measured item’s size when applying it to the grouped header or footer template, the collection view header or footer view, and the collection view header or footer template in PreferredLayoutAttributesFittingAttributes of TemplatedCell2.

Issues Fixed

Fixes #33130

Validated the behaviour in the following platforms

• Android
• Windows ,
• iOS,
• MacOS

Output

iOS

Before	After
issue33130_iOS_before.mov	Issue33130_iOS_after.mov

macOS

Before	After
issue33130_mac_before.mov	Issue33130_mac_after.mov

[Copilot]

Pull request overview

This PR fixes a CollectionView issue on iOS and macOS where group headers and footers incorrectly inherit the size of the first measured item when the ItemSizingStrategy is set to MeasureFirstItem. The fix introduces a boolean flag isSupplementaryView to distinguish supplementary views (headers/footers) from regular item cells, ensuring headers/footers are always measured directly and never use the cached first-item measurement.

Key Changes:

• Added flag to prevent headers/footers from using the first-item measurement cache
• Updated measurement logic to conditionally apply cached measurements only to item cells
• Added comprehensive UI test to verify the fix works correctly

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/Controls/src/Core/Handlers/Items2/iOS/TemplatedCell2.cs	Adds isSupplementaryView flag and updates measurement logic to skip cached measurements for headers/footers
src/Controls/src/Core/Handlers/Items2/iOS/StructuredItemsViewController2.cs	Sets the isSupplementaryView flag when configuring supplementary views
src/Controls/src/Core/Handlers/Items2/iOS/ItemsViewController2.cs	Explicitly clears the isSupplementaryView flag for regular item cells
src/Controls/src/Core/Handlers/Items2/iOS/GroupableItemsViewController2.cs	Sets the isSupplementaryView flag for group headers and footers
src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue33130.cs	NUnit test that verifies header height remains constant when ItemSizingStrategy changes
src/Controls/tests/TestCases.HostApp/Issues/Issue33130.xaml	XAML test page with grouped CollectionView and button to switch sizing strategy
src/Controls/tests/TestCases.HostApp/Issues/Issue33130.xaml.cs	Code-behind with view models demonstrating the group header sizing issue

[Copilot]

The field isSupplementaryView does not follow C# naming conventions. Internal fields should use PascalCase with an underscore prefix (e.g., _isSupplementaryView) to be consistent with other fields in this class like _measureInvalidated, _needsArrange, _measuredSize, and _cachedConstraints.

[Copilot]

The class name GroupHeaderTestAnimalGroup is verbose and redundant. Following the naming pattern used in other test files (e.g., Issue12374Model), this should be named AnimalGroup or Issue33130AnimalGroup to be more concise while maintaining clarity.

[Copilot]

The class name GroupHeaderTestAnimal is verbose with redundant "GroupHeaderTest" prefix. Following the naming pattern used in other test files (e.g., Issue12374Model), this should be named Animal or Issue33130Animal to be more concise while maintaining clarity.

Suggested change

	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 ObservableCollection<Issue33130AnimalGroup> Animals { get; set; }
	public GroupedAnimalsViewModel()
	{
	Animals = new ObservableCollection<Issue33130AnimalGroup>
	{
	new Issue33130AnimalGroup("Bears")
	{
	new Issue33130Animal { Name = "Grizzly Bear", Location = "North America", ImageUrl = "bear.jpg" },
	new Issue33130Animal { Name = "Polar Bear", Location = "Arctic", ImageUrl = "bear.jpg" },
	},
	new Issue33130AnimalGroup("Monkeys")
	{
	new Issue33130Animal { Name = "Baboon", Location = "Africa", ImageUrl = "monkey.jpg" },
	new Issue33130Animal { Name = "Capuchin Monkey", Location = "South America", ImageUrl = "monkey.jpg" },
	new Issue33130Animal { Name = "Spider Monkey", Location = "Central America", ImageUrl = "monkey.jpg" },
	},
	new Issue33130AnimalGroup("Elephants")
	{
	new Issue33130Animal { Name = "African Elephant", Location = "Africa", ImageUrl = "elephant.jpg" },
	new Issue33130Animal { Name = "Asian Elephant", Location = "Asia", ImageUrl = "elephant.jpg" },
	}
	};
	}
	}
	public class Issue33130AnimalGroup : ObservableCollection<Issue33130Animal>
	{
	public string Name { get; set; }
	public Issue33130AnimalGroup(string name) : base()
	{
	Name = name;
	}
	}
	public class Issue33130Animal

[Copilot]

The assertion uses Is.EqualTo(0) which expects the height difference to be exactly zero. However, the comment on line 37 mentions "Allow for small rounding differences". This is inconsistent. If you want to allow for small rounding differences, the assertion should use a tolerance (e.g., Is.EqualTo(0).Within(1) or Is.LessThanOrEqualTo(1)) instead of requiring exact equality. As written, the test may fail due to minor floating-point precision differences across platforms.

Suggested change

	Assert.That(heightDifference, Is.EqualTo(0),
	Assert.That(heightDifference, Is.EqualTo(0).Within(5),

[Copilot]

The comment "Ensure this cell is treated as a regular item cell (not a supplementary view)" is helpful, but it would be more informative to explain why this is necessary. Consider expanding it to: "Ensure this cell is treated as a regular item cell (not a supplementary view) so it can use the cached first-item measurement for MeasureFirstItem strategy" to provide better context about the fix's purpose.

Suggested change

	// Ensure this cell is treated as a regular item cell (not a supplementary view)
	// Ensure this cell is treated as a regular item cell (not a supplementary view)
	// so it can use the cached first-item measurement for MeasureFirstItem strategy

[PureWeen]

/backport to release/10.0.1xx-sr2

[github-actions]

Started backporting to release/10.0.1xx-sr2 (link to workflow run)

[PureWeen]

Set this to just CollectionView if this is passing on both CV1 and CV2

[NanthiniMahalingam]

Set this to just CollectionView if this is passing on both CV1 and CV2

Hi @PureWeen ,
I have changed CollectionView2 to CollectionView to ensure the test case passes on both CV1 and CV2.

[PureWeen]

/azp run

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