Fix s_currentTweener++ in AnimationExtensions is not thread-safe, producing duplicate animation IDs

src/Controls/src/Core/AnimationExtensions.cs

@@ -28,6 +28,7 @@
 using System;
 using System.Collections.Concurrent;
 using System.Collections.Generic;
+using System.Threading;
 using Microsoft.Maui.Animations;
 using Microsoft.Maui.Controls.Internals;
 using Microsoft.Maui.Dispatching;
@@ -64,7 +65,7 @@ static AnimationExtensions()
 
 		public static int Add(this IAnimationManager animationManager, Action<double> step)
 		{
-			var id = s_currentTweener++;
+			var id = Interlocked.Increment(ref s_currentTweener);
 			var animation = new Animation
 			{
 				Name = $"{id}",
@@ -84,7 +85,7 @@ public static int Add(this IAnimationManager animationManager, Action<double> st
 
 		public static int Insert(this IAnimationManager animationManager, Func<long, bool> step)
 		{
-			var id = s_currentTweener++;
+			var id = Interlocked.Increment(ref s_currentTweener);
 			Animation animation = null;
 			animation = new TweenerAnimation(step)
 			{

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

@@ -0,0 +1,88 @@
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using Microsoft.Maui.Animations;
+using Xunit;
+
+namespace Microsoft.Maui.Controls.Core.UnitTests
+{
+	public class AnimationExtensionsThreadSafetyTests
+	{
+		[Fact]
+		public void Add_ConcurrentCalls_ProducesUniqueIds()
+		{
+			// Exercises the REAL AnimationExtensions.Add() from multiple threads.
+			// If the fix is reverted (s_currentTweener++ instead of Interlocked.Increment),
+			// this test will fail with duplicate IDs under contention.
+			var manager = new NoOpAnimationManager();
+			const int threadCount = 50;
+			const int callsPerThread = 200;
+			var ids = new ConcurrentBag<int>();
+
+			var tasks = new Task[threadCount];
+			for (int i = 0; i < threadCount; i++)
+			{
+				tasks[i] = Task.Run(() =>
+				{
+					for (int j = 0; j < callsPerThread; j++)
+					{
+						int id = manager.Add(_ => { });
+						ids.Add(id);
+					}
+				});
+			}
+
+			Task.WaitAll(tasks);
+
+			int expectedCount = threadCount * callsPerThread;
+			var uniqueIds = new HashSet<int>(ids);
+
+			Assert.Equal(expectedCount, ids.Count);
+			Assert.Equal(expectedCount, uniqueIds.Count);
+		}
+
+		[Fact]
+		public void Insert_ConcurrentCalls_ProducesUniqueIds()
+		{
+			// Same as above but exercises AnimationExtensions.Insert().
+			var manager = new NoOpAnimationManager();
+			const int threadCount = 50;
+			const int callsPerThread = 200;
+			var ids = new ConcurrentBag<int>();
+
+			var tasks = new Task[threadCount];
+			for (int i = 0; i < threadCount; i++)
+			{
+				tasks[i] = Task.Run(() =>
+				{
+					for (int j = 0; j < callsPerThread; j++)
+					{
+						int id = manager.Insert(_ => true);
+						ids.Add(id);
+					}
+				});
+			}
+
+			Task.WaitAll(tasks);
+
+			int expectedCount = threadCount * callsPerThread;
+			var uniqueIds = new HashSet<int>(ids);
+
+			Assert.Equal(expectedCount, ids.Count);
+			Assert.Equal(expectedCount, uniqueIds.Count);
+		}
+
+		/// <summary>
+		/// Minimal no-op IAnimationManager so AnimationExtensions.Add/Insert can run
+		/// without requiring a platform ticker (which would block or crash off-thread).
+		/// </summary>
+		sealed class NoOpAnimationManager : IAnimationManager
+		{
+			public ITicker Ticker => null!;
+			public double SpeedModifier { get; set; } = 1;
+			public bool AutoStartTicker { get; set; }
+			public void Add(Microsoft.Maui.Animations.Animation animation) { }
+			public void Remove(Microsoft.Maui.Animations.Animation animation) { }
+		}
+	}
+}