[Android] Fixed CollectionView reordering last item

src/Controls/src/Core/Handlers/Items/Android/Adapters/ReorderableItemsViewAdapter.cs

@@ -63,10 +63,14 @@ public bool OnItemMove(int fromPosition, int toPosition)
 			}
 			else if (itemsView.ItemsSource is IList list)
 			{
-				var fromItem = list[fromPosition];
+				var hasHeader = itemsSource.HasHeader;
+				var fromPositionInSource = hasHeader ? fromPosition - 1 : fromPosition;
+				var toPositionInSource = hasHeader ? toPosition - 1 : toPosition;
+
+				var fromItem = list[fromPositionInSource];
 				SetObserveChanges(itemsSource, false);
-				list.RemoveAt(fromPosition);
-				list.Insert(toPosition, fromItem);
+				list.RemoveAt(fromPositionInSource);
+				list.Insert(toPositionInSource, fromItem);
 				NotifyItemMoved(fromPosition, toPosition);
 				SetObserveChanges(itemsSource, true);
 				itemsView.SendReorderCompleted();

src/Controls/tests/TestCases.HostApp/Issues/Issue17823.xaml

@@ -0,0 +1,39 @@
+<?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"
+             xmlns:local="clr-namespace:Maui.Controls.Sample.Issues"
+             x:Class="Maui.Controls.Sample.Issues.Issue17823"
+             Title="Issue 17823 - CollectionView reorder with header">
+    <VerticalStackLayout Padding="16" Spacing="12">
+        <CollectionView x:Name="ReorderCollectionView"
+                        AutomationId="ReorderCollectionView"
+                        ItemsSource="{Binding Items}"
+                        CanReorderItems="True"
+                        SelectionMode="None">
+            <CollectionView.Header>
+                <Grid Padding="12" BackgroundColor="#EEEEEE">
+                    <Label Text="Header Content"
+                           AutomationId="HeaderLabel"
+                           FontAttributes="Bold"/>
+                </Grid>
+            </CollectionView.Header>
+            <CollectionView.ItemTemplate>
+                <DataTemplate x:DataType="local:Issue17823+ReorderItem">
+                    <Grid Padding="16"
+                          Margin="0,0,0,8"
+                          BackgroundColor="#FFFFFF"
+                          AutomationId="{Binding ContainerAutomationId}">
+                        <Label Text="{Binding Text}"
+                               FontSize="18"
+                               HorizontalOptions="Center"
+                               AutomationId="{Binding LabelAutomationId}"/>
+                    </Grid>
+                </DataTemplate>
+            </CollectionView.ItemTemplate>
+        </CollectionView>
+
+        <Label x:Name="ReorderStatusLabel"
+               AutomationId="ReorderStatusLabel"
+               FontAttributes="Bold"/>
+    </VerticalStackLayout>
+</ContentPage>

src/Controls/tests/TestCases.HostApp/Issues/Issue17823.xaml.cs

@@ -0,0 +1,56 @@
+using System;
+using System.Collections.ObjectModel;
+using System.Linq;
+
+namespace Maui.Controls.Sample.Issues;
+
+[Issue(IssueTracker.Github, 17823, "CollectionView reordering last item succeeds when header is present", PlatformAffected.Android)]
+public partial class Issue17823 : ContentPage
+{
+	public ObservableCollection<ReorderItem> Items { get; }
+
+	public Issue17823()
+	{
+		InitializeComponent();
+
+		Items = new ObservableCollection<ReorderItem>
+		{
+			new ReorderItem(0, "Item 1"),
+			new ReorderItem(1, "Item 2"),
+			new ReorderItem(2, "Item 3"),
+			new ReorderItem(3, "Item 4"),
+		};
+
+		BindingContext = this;
+
+		ReorderCollectionView.ReorderCompleted += OnReorderCompleted;
+
+		UpdateStatusLabel();
+	}
+
+	void OnReorderCompleted(object sender, EventArgs e)
+	{
+		UpdateStatusLabel();
+	}
+
+	void UpdateStatusLabel()
+	{
+		ReorderStatusLabel.Text = string.Join(", ", Items.Select(item => item.Text));
+	}
+
+	public class ReorderItem
+	{
+		public ReorderItem(int index, string text)
+		{
+			Text = text;
+			ContainerAutomationId = $"ReorderItem{index}";
+			LabelAutomationId = $"ReorderItemLabel{index}";
+		}
+
+		public string Text { get; set; }
+
+		public string ContainerAutomationId { get; }
+
+		public string LabelAutomationId { get; }
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue17823.cs

@@ -0,0 +1,63 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+public class Issue17823 : _IssuesUITest
+{
+	public Issue17823(TestDevice device) : base(device)
+	{
+	}
+
+	public override string Issue => "CollectionView reordering last item succeeds when header is present";
+
+	[Test]
+	[Category(UITestCategories.CollectionView)]
+	public void ReorderingItemToEndWithHeaderDoesNotCrash()
+	{
+		// Verify header is present
+		App.WaitForElement("HeaderLabel");
+
+		// Verify all items are present
+		App.WaitForElement("ReorderItem0");
+		App.WaitForElement("ReorderItem3");
+
+		// Verify initial state
+		var initialText = App.FindElement("ReorderStatusLabel").GetText();
+		Assert.That(initialText, Is.EqualTo("Item 1, Item 2, Item 3, Item 4"));
+
+		// The bug: dragging first item to the end would crash with header present
+		// This is because adapter indices include header (0=header, 1=item0, 2=item1, etc.)
+		// but item source indices don't (0=item0, 1=item1, etc.)
+		App.DragAndDrop("ReorderItem0", "ReorderItem3");
+
+		// Verify reorder succeeded without crash by checking status label updated
+		var afterDrag = App.FindElement("ReorderStatusLabel").GetText();
+		Assert.That(afterDrag, Is.EqualTo("Item 2, Item 3, Item 4, Item 1"));
+	}
+
+	[Test]
+	[Category(UITestCategories.CollectionView)]
+	public void ReorderingLastItemWithHeaderDoesNotCrash()
+	{
+		// Verify header is present
+		App.WaitForElement("HeaderLabel");
+
+		// Verify items are present
+		App.WaitForElement("ReorderItem0");
+		App.WaitForElement("ReorderItem3");
+
+		// Verify initial state
+		var initialText = App.FindElement("ReorderStatusLabel").GetText();
+		Assert.That(initialText, Is.EqualTo("Item 1, Item 2, Item 3, Item 4"));
+
+		// Test dragging last item to first position
+		// This tests the opposite direction
+		App.DragAndDrop("ReorderItem3", "ReorderItem0");
+
+		// Verify reorder succeeded without crash
+		var afterDrag = App.FindElement("ReorderStatusLabel").GetText();
+		Assert.That(afterDrag, Is.EqualTo("Item 4, Item 1, Item 2, Item 3"));
+	}
+}