Fixing regression in Picker behavior in 8.0.60 (#23369)

* Fixing regression in Picker behavior when inserting at the beginning and incrementing SelectedIndex.

* Fixing off by one error in Picker item removing logic

* Working on picker unit tests for adding and removing items and how it interacts with SelectedItem and SelectedIndex.

* Updating Picker remove index calculation logic to make it easier to read. Added unit tests for Picker to check SelectedIndex and SelectedItem when adding/removing single and multiple items.

src/Controls/src/Core/Picker/Picker.cs

@@ -302,19 +302,36 @@ void CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
 
 		void AddItems(NotifyCollectionChangedEventArgs e)
 		{
-			int index = e.NewStartingIndex < 0 ? Items.Count : e.NewStartingIndex;
+			int insertIndex = e.NewStartingIndex < 0 ? Items.Count : e.NewStartingIndex;
+			int index = insertIndex;
 			foreach (object newItem in e.NewItems)
 				((LockableObservableListWrapper)Items).InternalInsert(index++, GetDisplayMember(newItem));
-			if (index <= SelectedIndex)
+			if (insertIndex <= SelectedIndex)
 				UpdateSelectedItem(SelectedIndex);
 		}
 
 		void RemoveItems(NotifyCollectionChangedEventArgs e)
 		{
-			int index = e.OldStartingIndex < Items.Count ? e.OldStartingIndex : Items.Count;
+			int removeStart;
+			// Items are removed in reverse order, so index starts at the index of the last item to remove
+			int index;
+
+			if (e.OldStartingIndex < Items.Count)
+			{
+				// Remove e.OldItems.Count items starting at e.OldStartingIndex
+				removeStart = e.OldStartingIndex;
+				index = e.OldStartingIndex + e.OldItems.Count - 1;
+			}
+			else
+			{
+				// Remove e.OldItems.Count items at the end when e.OldStartingIndex is past the end of the Items collection
+				removeStart = Items.Count - e.OldItems.Count;
+				index = Items.Count - 1;
+			}
+
 			foreach (object _ in e.OldItems)
 				((LockableObservableListWrapper)Items).InternalRemoveAt(index--);
-			if (index <= SelectedIndex)
+			if (removeStart <= SelectedIndex)
 			{
 				ClampSelectedIndex();
 			}

src/Controls/tests/Core.UnitTests/PickerTests.cs

@@ -1,6 +1,10 @@
 using System;
+using System.Collections.Generic;
 using System.Collections.ObjectModel;
+using System.Collections.Specialized;
+using System.ComponentModel;
 using System.Globalization;
+using System.Linq;
 using Microsoft.Maui.Graphics;
 using Xunit;
 
@@ -57,6 +61,42 @@ public object ConvertBack(object value, Type targetType, object parameter, Cultu
 			}
 		}
 
+		class ObservableRangeCollection<T> : ObservableCollection<T>
+		{
+			static readonly PropertyChangedEventArgs CountChangedArgs = new(nameof(Count));
+			static readonly PropertyChangedEventArgs IndexerChangedArgs = new("Item[]");
+
+			public void InsertRange(int index, IEnumerable<T> items)
+			{
+				CheckReentrancy();
+				int currIndex = index;
+				foreach (T item in items)
+				{
+					Items.Insert(currIndex++, item);
+				}
+
+				OnPropertyChanged(CountChangedArgs);
+				OnPropertyChanged(IndexerChangedArgs);
+				OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, items.ToList(), index));
+			}
+
+			public void RemoveRange(int index, int count)
+			{
+				CheckReentrancy();
+				T[] removeItems = new T[count];
+				for (int i = 0; i < count; i++)
+				{
+					// Always remove at index, since removing each item at index shifts the next item to that spot
+					removeItems[i] = Items[index];
+					Items.RemoveAt(index);
+				}
+
+				OnPropertyChanged(CountChangedArgs);
+				OnPropertyChanged(IndexerChangedArgs);
+				OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Remove, removeItems, index));
+			}
+		}
+
 		[Fact]
 		public void TestSetSelectedIndexOnNullRows()
 		{
@@ -338,6 +378,89 @@ public void TestItemsSourceCollectionChangedRemove()
 			Assert.Equal("Ringo", picker.Items[1]);
 		}
 
+		[Theory]
+		[InlineData(0, new string[] { "George" })]
+		[InlineData(1, new string[] { "George" })]
+		[InlineData(2, new string[] { "George" })]
+		[InlineData(3, new string[] { "George" })]
+		[InlineData(0, new string[] { "George", "Pete" })]
+		[InlineData(1, new string[] { "George", "Pete" })]
+		[InlineData(2, new string[] { "George", "Pete" })]
+		[InlineData(3, new string[] { "George", "Pete" })]
+		public void TestItemsSourceCollectionChangedInsertBeforeSelected(int insertionIndex, string[] insertNames)
+		{
+			var items = new ObservableRangeCollection<object>
+			{
+				new { Name = "John" },
+				new { Name = "Paul" },
+				new { Name = "Ringo" }
+			};
+			var picker = new Picker
+			{
+				ItemDisplayBinding = new Binding("Name"),
+				ItemsSource = items,
+				SelectedIndex = 1
+			};
+			items.InsertRange(insertionIndex, insertNames.Select(name => new { Name = name }));
+			Assert.Equal(3 + insertNames.Length, picker.Items.Count);
+			Assert.Equal(1, picker.SelectedIndex);
+			Assert.Equal(items[1], picker.SelectedItem);
+		}
+
+		[Theory]
+		[InlineData(0, 1)]
+		[InlineData(1, 1)]
+		[InlineData(2, 1)]
+		[InlineData(0, 2)]
+		[InlineData(1, 2)]
+		[InlineData(2, 2)]
+		public void TestItemsSourceCollectionChangedRemoveBeforeSelected(int removeIndex, int removeCount)
+		{
+			var items = new ObservableRangeCollection<object>
+			{
+				new { Name = "John" },
+				new { Name = "Paul" },
+				new { Name = "Ringo" },
+				new { Name = "George" }
+			};
+			var picker = new Picker
+			{
+				ItemDisplayBinding = new Binding("Name"),
+				ItemsSource = items,
+				SelectedIndex = 1
+			};
+			items.RemoveRange(removeIndex, removeCount);
+
+			Assert.Equal(4 - removeCount, picker.Items.Count);
+			Assert.Equal(1, picker.SelectedIndex);
+			Assert.Equal(items[1], picker.SelectedItem);
+		}
+
+		[Theory]
+		[InlineData(1)]
+		[InlineData(2)]
+		public void TestItemsSourceCollectionChangedRemoveAtEndSelected(int removeCount)
+		{
+			var items = new ObservableRangeCollection<object>
+			{
+				new { Name = "John" },
+				new { Name = "Paul" },
+				new { Name = "Ringo" },
+				new { Name = "George" }
+			};
+			var picker = new Picker
+			{
+				ItemDisplayBinding = new Binding("Name"),
+				ItemsSource = items,
+				SelectedIndex = 4 - removeCount
+			};
+			items.RemoveRange(4 - removeCount, removeCount);
+
+			Assert.Equal(4 - removeCount, picker.Items.Count);
+			Assert.Equal(items.Count - 1, picker.SelectedIndex);
+			Assert.Equal(items[^1], picker.SelectedItem);
+		}
+
 		[Fact]
 		public void TestSelectedIndexAssignedItemsSourceCollectionChangedAppend()
 		{