[Windows] Fixed CollectionView throws NRE when value of IsGrouped property is changed to false

.github/copilot-instructions.md

@@ -200,20 +200,6 @@ git commit -m "Fix: Description of the change"
 - `.github/instructions/android.instructions.md` - Android handler implementation
 - `.github/instructions/xaml-unittests.instructions.md` - XAML unit test guidelines
 
-### Opening PRs
-
-All PRs are required to have this at the top of the description:
-
-```
-<!-- Please let the below note in for people that find this PR -->
-> [!NOTE]
-> Are you waiting for the changes in this PR to be merged?
-> It would be very helpful if you could [test the resulting artifacts](https://github.com/dotnet/maui/wiki/Testing-PR-Builds) from this PR and let us know in a comment if this change resolves your issue. Thank you!
-```
-
-Always put that at the top, without the block quotes. Without it, users will NOT be able to try the PR and your work will have been in vain!
-
-
 
 ## Custom Agents and Skills
 

.github/skills/pr-finalize/SKILL.md

@@ -127,16 +127,10 @@ Examples:
 ## Description Requirements
 
 PR description should:
-1. Start with the required NOTE block (so users can test PR artifacts)
-2. Include the base sections from `.github/PULL_REQUEST_TEMPLATE.md` ("Description of Change" and "Issues Fixed"). The skill adds additional structured fields (Root cause, Fix, Key insight, etc.) as recommended enhancements for better agent context.
-3. Match the actual implementation
+1. Include the base sections from `.github/PULL_REQUEST_TEMPLATE.md` ("Description of Change" and "Issues Fixed"). The skill adds additional structured fields (Root cause, Fix, Key insight, etc.) as recommended enhancements for better agent context.
+2. Match the actual implementation
 
 ```markdown
-<!-- Please let the below note in for people that find this PR -->
-> [!NOTE]
-> Are you waiting for the changes in this PR to be merged?
-> It would be very helpful if you could [test the resulting artifacts](https://github.com/dotnet/maui/wiki/Testing-PR-Builds) from this PR and let us know in a comment if this change resolves your issue. Thank you!
-
 ### Description of Change
 [Must match actual implementation]
 
@@ -229,11 +223,6 @@ Example: "Before: Safe area applied by default (opt-out). After: Only views impl
 Use structured template only when existing description is inadequate:
 
 ```markdown
-<!-- Please let the below note in for people that find this PR -->
-> [!NOTE]
-> Are you waiting for the changes in this PR to be merged?
-> It would be very helpful if you could [test the resulting artifacts](https://github.com/dotnet/maui/wiki/Testing-PR-Builds) from this PR and let us know in a comment if this change resolves your issue. Thank you!
-
 ### Root Cause
 
 [Why the bug occurred - be specific about the code path]

.github/skills/pr-finalize/references/complete-example.md

@@ -9,10 +9,6 @@ This example shows a PR description optimized for future agent success.
 
 ## Description
 ```markdown
-> [!NOTE]
-> Are you waiting for the changes in this PR to be merged?
-> It would be very helpful if you could [test the resulting artifacts](https://github.com/dotnet/maui/wiki/Testing-PR-Builds) from this PR and let us know in a comment if this change resolves your issue. Thank you!
-
 ### Root Cause
 
 In `MauiView.GetAdjustedSafeAreaInsets()` on iOS, views that don't implement `ISafeAreaView` or `ISafeAreaView2` (such as `ContentPresenter`, `Border`) were falling through to return `baseSafeArea`. This applied full device safe area insets to views that never opted into safe area handling, causing double-padding when used inside ControlTemplates.

eng/pipelines/common/ui-tests-steps.yml

@@ -186,6 +186,8 @@ steps:
 - task: PublishBuildArtifacts@1
   condition: always()
   displayName: publish artifacts
+  inputs:
+    artifactName: drop-$(System.StageName)-$(System.JobName)-$(System.JobAttempt)
 
 # Enable Notification Center re-enabled only for catalyst
 - ${{ if eq(parameters.platform, 'catalyst')}}:

src/Controls/src/Core/Platform/Windows/CollectionView/TemplatedItemSourceFactory.cs

@@ -1,4 +1,5 @@
 #nullable disable
+using System;
 using System.Collections;
 using System.Collections.Specialized;
 
@@ -9,6 +10,11 @@ internal static class TemplatedItemSourceFactory
 		internal static object Create(IEnumerable itemsSource, DataTemplate itemTemplate, BindableObject container,
 			double? itemHeight = null, double? itemWidth = null, Thickness? itemSpacing = null, IMauiContext mauiContext = null)
 		{
+			if (itemsSource is null)
+			{
+				return Array.Empty<object>();
+			}
+
 			switch (itemsSource)
 			{
 				case IList observable when itemsSource is INotifyCollectionChanged:

src/Controls/tests/TestCases.HostApp/Issues/Issue17864.xaml

@@ -0,0 +1,46 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<controls:TestContentPage
+  xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+  xmlns:controls="clr-namespace:Maui.Controls.Sample.Issues"
+  xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+  x:Class="Maui.Controls.Sample.Issues.Issue17864">
+  <Grid>
+    <Grid.RowDefinitions>
+      <RowDefinition Height="Auto"/>
+      <RowDefinition Height="Auto"/>
+      <RowDefinition Height="*"/>
+    </Grid.RowDefinitions>
+
+    <Button Clicked="Button_Clicked"
+        AutomationId="Button"
+            Text="Disable Grouping"/>
+
+    <Button Grid.Row="1"
+            Clicked="EnableGroupingOnlyButton_Clicked"
+            AutomationId="EnableGroupingOnlyButton"
+            Text="Enable Grouping Without Regrouping"/>
+
+    <CollectionView Grid.Row="2"
+        AutomationId="CollectionView"
+                    ItemsSource="{Binding Items}"
+                    IsGrouped="{Binding IsGrouped}">
+      <CollectionView.GroupHeaderTemplate>
+        <DataTemplate>
+          <Label Margin="2"
+              FontSize="Medium"
+              Text="{Binding ItemText}"
+              HorizontalOptions="Fill"
+              TextColor="LightBlue"/>
+        </DataTemplate>
+      </CollectionView.GroupHeaderTemplate>
+      <CollectionView.ItemTemplate>
+        <DataTemplate>
+          <Label Margin="15,2"
+              Text="{Binding ItemText}"
+              HorizontalOptions="Fill"
+              TextColor="Gray"/>
+        </DataTemplate>
+      </CollectionView.ItemTemplate>
+    </CollectionView>
+  </Grid>
+</controls:TestContentPage>

src/Controls/tests/TestCases.HostApp/Issues/Issue17864.xaml.cs

@@ -0,0 +1,122 @@
+using System.Collections.ObjectModel;
+using System.ComponentModel;
+
+namespace Maui.Controls.Sample.Issues
+{
+	[Issue(IssueTracker.Github, 17864, "[Windows] CollectionView throws NRE when value of IsGrouped property is changed to false",
+		PlatformAffected.UWP)]
+	public partial class Issue17864 : TestContentPage
+	{
+		readonly Issue17864_ItemListViewModel _ItemListViewModel;
+		protected override void Init() { }
+
+		public Issue17864()
+		{
+			InitializeComponent();
+			_ItemListViewModel = new Issue17864_ItemListViewModel();
+			BindingContext = _ItemListViewModel;
+		}
+
+		private void Button_Clicked(object sender, EventArgs e)
+		{
+			_ItemListViewModel.IsGrouped = !_ItemListViewModel.IsGrouped;
+		}
+
+		private void EnableGroupingOnlyButton_Clicked(object sender, EventArgs e)
+		{
+			_ItemListViewModel.EnableGroupingWithoutRegrouping();
+		}
+	}
+
+	class Issue17864_GroupViewModel : ObservableCollection<Issue17864_ItemViewModel>, Issue17864_IItemViewModel
+	{
+
+		public Issue17864_GroupViewModel(int groupIndex)
+		{
+			ItemText = $"Group #{groupIndex}";
+		}
+
+		public string ItemText { get; }
+	}
+
+	interface Issue17864_IItemViewModel
+	{
+		string ItemText { get; }
+	}
+
+	class Issue17864_ItemListViewModel : INotifyPropertyChanged
+	{
+		public event PropertyChangedEventHandler PropertyChanged;
+
+		ObservableCollection<Issue17864_IItemViewModel> items = new();
+
+		private bool isGrouped = true;
+
+
+		public Issue17864_ItemListViewModel()
+		{
+			Populate();
+		}
+
+		private void Populate()
+		{
+			items.Clear();
+			if (isGrouped)
+			{
+				int groupIndex = 0;
+				items.Add(GetGroup(groupIndex++, 2));
+				items.Add(GetGroup(groupIndex++, 1));
+				items.Add(GetGroup(groupIndex++, 3));
+				items.Add(GetGroup(groupIndex++, 2));
+			}
+			else
+			{
+				int itemIndex = 0;
+				items.Add(new Issue17864_ItemViewModel(itemIndex++));
+				items.Add(new Issue17864_ItemViewModel(itemIndex++));
+				items.Add(new Issue17864_ItemViewModel(itemIndex++));
+			}
+		}
+
+		private Issue17864_IItemViewModel GetGroup(int groupIndex, int itemCount)
+		{
+			var group = new Issue17864_GroupViewModel(groupIndex);
+			for (int i = 0; i < itemCount; i++)
+			{
+				group.Add(new Issue17864_ItemViewModel(i));
+			}
+			return group;
+		}
+
+		public ObservableCollection<Issue17864_IItemViewModel> Items => items;
+
+		public void EnableGroupingWithoutRegrouping()
+		{
+			isGrouped = true;
+			PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(IsGrouped)));
+		}
+
+		public bool IsGrouped
+		{
+			get => isGrouped;
+			set
+			{
+				isGrouped = value;
+				Populate();
+				PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(IsGrouped)));
+			}
+		}
+
+	}
+
+	class Issue17864_ItemViewModel : Issue17864_IItemViewModel
+	{
+
+		public Issue17864_ItemViewModel(int itemIndex)
+		{
+			ItemText = $"Item #{itemIndex}";
+		}
+
+		public string ItemText { get; }
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue17864.cs

@@ -0,0 +1,31 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues
+{
+	public class Issue17864 : _IssuesUITest
+	{
+		const string CollectionView = "CollectionView";
+		public Issue17864(TestDevice device) : base(device)
+		{
+		}
+
+		public override string Issue => "[Windows] CollectionView throws NRE when value of IsGrouped property is changed to false";
+
+		[Test]
+		[Category(UITestCategories.CollectionView)]
+		public void CollectionViewShouldNotCrashWhenIsGroupedChangesInBothDirections()
+		{
+			App.WaitForElement(CollectionView);
+
+			// true -> false
+			App.Tap("Button");
+			App.WaitForElement(CollectionView);
+
+			// false -> true while items remain flat
+			App.Tap("EnableGroupingOnlyButton");
+			App.WaitForElement(CollectionView);
+		}
+	}
+}