feat: Implement AutoReverse support for Timeline animations and Storyboard containers

src/SamplesApp/UITests.Shared/UITests.Shared.projitems

@@ -5290,6 +5290,10 @@
       <SubType>Designer</SubType>
       <Generator>MSBuild:Compile</Generator>
     </Page>
+    <Page Include="$(MSBuildThisFileDirectory)Windows_UI_Xaml_Media_Animation\DoubleAnimation_AutoReverse.xaml">
+      <SubType>Designer</SubType>
+      <Generator>MSBuild:Compile</Generator>
+    </Page>
     <Page Include="$(MSBuildThisFileDirectory)Windows_UI_Xaml_Media_Animation\DoubleAnimation_RenderTransformOrigin.xaml">
       <SubType>Designer</SubType>
       <Generator>MSBuild:Compile</Generator>
@@ -8932,6 +8936,9 @@
     <Compile Include="$(MSBuildThisFileDirectory)Windows_UI_Xaml_Media_Animation\DoubleAnimation_Opacity_TextBlock.xaml.cs">
       <DependentUpon>DoubleAnimation_Opacity_TextBlock.xaml</DependentUpon>
     </Compile>
+    <Compile Include="$(MSBuildThisFileDirectory)Windows_UI_Xaml_Media_Animation\DoubleAnimation_AutoReverse.xaml.cs">
+      <DependentUpon>DoubleAnimation_AutoReverse.xaml</DependentUpon>
+    </Compile>
     <Compile Include="$(MSBuildThisFileDirectory)Windows_UI_Xaml_Media_Animation\DoubleAnimation_RenderTransformOrigin.xaml.cs">
       <DependentUpon>DoubleAnimation_RenderTransformOrigin.xaml</DependentUpon>
     </Compile>

src/SamplesApp/UITests.Shared/Windows_UI_Xaml_Media_Animation/DoubleAnimation_AutoReverse.xaml

@@ -0,0 +1,58 @@
+<UserControl
+	x:Class="GenericApp.Views.Content.UITests.Animations.DoubleAnimation_AutoReverse"
+	xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+	xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+	xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+	xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+	mc:Ignorable="d"
+	d:DesignHeight="600"
+	d:DesignWidth="400">
+
+	<Grid>
+		<Grid.RowDefinitions>
+			<RowDefinition Height="Auto" />
+			<RowDefinition Height="Auto" />
+			<RowDefinition Height="*" />
+		</Grid.RowDefinitions>
+
+		<StackPanel Grid.Row="0" Orientation="Horizontal" Spacing="10" Margin="10">
+			<Button x:Name="StartButton" Content="Start" Click="StartAnimation" />
+			<Button x:Name="StopButton" Content="Stop" Click="StopAnimation" />
+		</StackPanel>
+
+		<TextBlock Grid.Row="1" x:Name="StatusText" Text="Click Start to begin animation" Margin="10" />
+
+		<Grid Grid.Row="2" Margin="20">
+			<Rectangle x:Name="AnimatedRectangle"
+				Width="50"
+				Height="50"
+				Fill="{ThemeResource SystemAccentColorBrush}"
+				HorizontalAlignment="Left"
+				VerticalAlignment="Top">
+				<Rectangle.RenderTransform>
+					<TranslateTransform x:Name="RectTransform" />
+				</Rectangle.RenderTransform>
+			</Rectangle>
+
+			<StackPanel VerticalAlignment="Bottom" Margin="10">
+				<TextBlock Text="AutoReverse=True" FontWeight="Bold" Margin="0,0,0,10" />
+				<TextBlock Text="Duration: 2 seconds" />
+				<TextBlock Text="From: 0, To: 300" />
+				<TextBlock Text="Expected: Move right, then back to start" TextWrapping="Wrap" />
+			</StackPanel>
+		</Grid>
+
+		<Grid.Resources>
+			<Storyboard x:Key="MoveStoryboard">
+				<DoubleAnimation
+					Storyboard.TargetName="RectTransform"
+					Storyboard.TargetProperty="X"
+					From="0"
+					To="300"
+					Duration="0:0:2"
+					AutoReverse="True"
+					FillBehavior="HoldEnd" />
+			</Storyboard>
+		</Grid.Resources>
+	</Grid>
+</UserControl>

src/SamplesApp/UITests.Shared/Windows_UI_Xaml_Media_Animation/DoubleAnimation_AutoReverse.xaml.cs

@@ -0,0 +1,42 @@
+using System;
+using Microsoft.UI.Xaml;
+using Microsoft.UI.Xaml.Controls;
+using Microsoft.UI.Xaml.Media.Animation;
+using Uno.UI.Samples.Controls;
+
+namespace GenericApp.Views.Content.UITests.Animations
+{
+	[Sample("Animations", Description = "Demonstrates AutoReverse functionality. The rectangle should move right then back to the start position.")]
+	public sealed partial class DoubleAnimation_AutoReverse : UserControl
+	{
+		private Storyboard _storyboard;
+
+		public DoubleAnimation_AutoReverse()
+		{
+			this.InitializeComponent();
+		}
+
+		private void StartAnimation(object sender, RoutedEventArgs e)
+		{
+			if (_storyboard == null)
+			{
+				_storyboard = (Storyboard)Resources["MoveStoryboard"];
+				_storyboard.Completed += OnStoryboardCompleted;
+			}
+
+			StatusText.Text = "Animation started...";
+			_storyboard.Begin();
+		}
+
+		private void StopAnimation(object sender, RoutedEventArgs e)
+		{
+			_storyboard?.Stop();
+			StatusText.Text = "Animation stopped";
+		}
+
+		private void OnStoryboardCompleted(object sender, object e)
+		{
+			StatusText.Text = "Animation completed - rectangle should be back at start position";
+		}
+	}
+}

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Media_Animation/Given_DoubleAnimation.cs

@@ -391,5 +391,146 @@ private static double GetTranslateY(TranslateTransform translate, bool isStillAn
 				// And, when the animation is completed, this native value is reset even for HoldEnd animation.
 				: translate.Y;
 #endif
+
+		[TestMethod]
+		[PlatformCondition(ConditionMode.Exclude, RuntimeTestPlatforms.NativeAndroid | RuntimeTestPlatforms.NativeIOS)]
+		public async Task When_AutoReverse_True()
+		{
+			var target = new TextBlock() { Text = "Test AutoReverse" };
+			WindowHelper.WindowContent = target;
+			await WindowHelper.WaitForIdle();
+			await WindowHelper.WaitForLoaded(target);
+
+			var transform = new TranslateTransform();
+			target.RenderTransform = transform;
+
+			var animation = new DoubleAnimation()
+			{
+				From = 0,
+				To = 100,
+				Duration = TimeSpan.FromMilliseconds(500),
+				AutoReverse = true,
+				FillBehavior = FillBehavior.HoldEnd,
+			};
+			Storyboard.SetTarget(animation, transform);
+			Storyboard.SetTargetProperty(animation, nameof(TranslateTransform.X));
+
+			var storyboard = new Storyboard();
+			storyboard.Children.Add(animation);
+
+			bool completed = false;
+			storyboard.Completed += (s, e) => completed = true;
+
+			storyboard.Begin();
+
+			// Wait for quarter of the animation (forward phase)
+			await Task.Delay(250);
+			var valueAt25Percent = transform.X;
+
+			// Wait for just past halfway (should be near the end of forward phase)
+			await Task.Delay(300);
+			var valueAt55Percent = transform.X;
+
+			// Wait for 75% of total duration (should be in reverse phase)
+			await Task.Delay(250);
+			var valueAt80Percent = transform.X;
+
+			// Wait for completion
+			await WindowHelper.WaitFor(() => completed, timeoutMS: 2000);
+
+			// Final value should be back at start (0) with HoldEnd
+			var finalValue = transform.X;
+
+			// Verify animation went forward then backward
+			Assert.IsTrue(valueAt25Percent > 0 && valueAt25Percent < 100,
+				$"At 25%, value should be between 0 and 100, got {valueAt25Percent}");
+			Assert.IsTrue(valueAt55Percent > valueAt25Percent,
+				$"At 55%, value should be greater than at 25%, got {valueAt55Percent} vs {valueAt25Percent}");
+			Assert.IsTrue(valueAt80Percent < valueAt55Percent,
+				$"At 80% (reverse phase), value should be less than at 55%, got {valueAt80Percent} vs {valueAt55Percent}");
+			Assert.IsTrue(Math.Abs(finalValue) < 10,
+				$"Final value should be close to 0, got {finalValue}");
+		}
+
+		[TestMethod]
+		[PlatformCondition(ConditionMode.Exclude, RuntimeTestPlatforms.NativeAndroid | RuntimeTestPlatforms.NativeIOS)]
+		public async Task When_AutoReverse_False()
+		{
+			var target = new TextBlock() { Text = "Test No AutoReverse" };
+			WindowHelper.WindowContent = target;
+			await WindowHelper.WaitForIdle();
+			await WindowHelper.WaitForLoaded(target);
+
+			var transform = new TranslateTransform();
+			target.RenderTransform = transform;
+
+			var animation = new DoubleAnimation()
+			{
+				From = 0,
+				To = 100,
+				Duration = TimeSpan.FromMilliseconds(500),
+				AutoReverse = false,
+				FillBehavior = FillBehavior.HoldEnd,
+			};
+			Storyboard.SetTarget(animation, transform);
+			Storyboard.SetTargetProperty(animation, nameof(TranslateTransform.X));
+
+			var storyboard = new Storyboard();
+			storyboard.Children.Add(animation);
+
+			bool completed = false;
+			storyboard.Completed += (s, e) => completed = true;
+
+			storyboard.Begin();
+
+			// Wait for completion
+			await WindowHelper.WaitFor(() => completed, timeoutMS: 2000);
+
+			// Final value should stay at end (100) with HoldEnd and no AutoReverse
+			var finalValue = transform.X;
+			Assert.IsTrue(Math.Abs(finalValue - 100) < 10,
+				$"Final value should be close to 100, got {finalValue}");
+		}
+
+		[TestMethod]
+		[PlatformCondition(ConditionMode.Exclude, RuntimeTestPlatforms.NativeAndroid | RuntimeTestPlatforms.NativeIOS)]
+		public async Task When_AutoReverse_WithRepeat()
+		{
+			var target = new TextBlock() { Text = "Test AutoReverse with Repeat" };
+			WindowHelper.WindowContent = target;
+			await WindowHelper.WaitForIdle();
+			await WindowHelper.WaitForLoaded(target);
+
+			var transform = new TranslateTransform();
+			target.RenderTransform = transform;
+
+			var animation = new DoubleAnimation()
+			{
+				From = 0,
+				To = 50,
+				Duration = TimeSpan.FromMilliseconds(250),
+				AutoReverse = true,
+				RepeatBehavior = new RepeatBehavior(2), // Repeat twice (4 total half-cycles)
+				FillBehavior = FillBehavior.HoldEnd,
+			};
+			Storyboard.SetTarget(animation, transform);
+			Storyboard.SetTargetProperty(animation, nameof(TranslateTransform.X));
+
+			var storyboard = new Storyboard();
+			storyboard.Children.Add(animation);
+
+			bool completed = false;
+			storyboard.Completed += (s, e) => completed = true;
+
+			storyboard.Begin();
+
+			// Wait for completion (2 repeats * 2 phases * 250ms = 1000ms)
+			await WindowHelper.WaitFor(() => completed, timeoutMS: 2500);
+
+			// Final value should be back at start (0)
+			var finalValue = transform.X;
+			Assert.IsTrue(Math.Abs(finalValue) < 10,
+				$"Final value should be close to 0 after repeating with AutoReverse, got {finalValue}");
+		}
 	}
 }

src/Uno.UI/Generated/3.0.0.0/Microsoft.UI.Xaml.Media.Animation/Timeline.cs

@@ -26,20 +26,7 @@ public double SpeedRatio
 		// Skipping already declared property FillBehavior
 		// Skipping already declared property Duration
 		// Skipping already declared property BeginTime
-#if __ANDROID__ || __IOS__ || __TVOS__ || IS_UNIT_TESTS || __WASM__ || __SKIA__ || __NETSTD_REFERENCE__
-		[global::Uno.NotImplemented("__ANDROID__", "__IOS__", "__TVOS__", "IS_UNIT_TESTS", "__WASM__", "__SKIA__", "__NETSTD_REFERENCE__")]
-		public bool AutoReverse
-		{
-			get
-			{
-				return (bool)this.GetValue(AutoReverseProperty);
-			}
-			set
-			{
-				this.SetValue(AutoReverseProperty, value);
-			}
-		}
-#endif
+		// Skipping already declared property AutoReverse
 #if __ANDROID__ || __IOS__ || __TVOS__ || IS_UNIT_TESTS || __WASM__ || __SKIA__ || __NETSTD_REFERENCE__
 		[global::Uno.NotImplemented("__ANDROID__", "__IOS__", "__TVOS__", "IS_UNIT_TESTS", "__WASM__", "__SKIA__", "__NETSTD_REFERENCE__")]
 		public static bool AllowDependentAnimations
@@ -54,14 +41,7 @@ public static bool AllowDependentAnimations
 			}
 		}
 #endif
-#if __ANDROID__ || __IOS__ || __TVOS__ || IS_UNIT_TESTS || __WASM__ || __SKIA__ || __NETSTD_REFERENCE__
-		[global::Uno.NotImplemented("__ANDROID__", "__IOS__", "__TVOS__", "IS_UNIT_TESTS", "__WASM__", "__SKIA__", "__NETSTD_REFERENCE__")]
-		public static global::Microsoft.UI.Xaml.DependencyProperty AutoReverseProperty { get; } =
-		Microsoft.UI.Xaml.DependencyProperty.Register(
-			nameof(AutoReverse), typeof(bool),
-			typeof(global::Microsoft.UI.Xaml.Media.Animation.Timeline),
-			new Microsoft.UI.Xaml.FrameworkPropertyMetadata(default(bool)));
-#endif
+		// Skipping already declared property AutoReverseProperty
 		// Skipping already declared property BeginTimeProperty
 		// Skipping already declared property DurationProperty
 		// Skipping already declared property FillBehaviorProperty

src/Uno.UI/UI/Xaml/Media/Animation/Timeline.animation.cs

@@ -36,6 +36,7 @@ public static class TraceProvider
 			private int _replayCount = 1;
 			private T? _startingValue;
 			private T? _endValue;
+			private bool _isReversing = false; // Track if we're in the reverse phase of AutoReverse
 
 			// Initialize the field with zero capacity, as it may stay empty more often than it is being used.
 			private readonly CompositeDisposable _subscriptions = new CompositeDisposable(0);
@@ -61,6 +62,7 @@ private TimelineState State
 			private TimeSpan? BeginTime => _owner?.BeginTime;
 			private Duration Duration => _owner?.Duration ?? default(Duration);
 			private FillBehavior FillBehavior => _owner?.FillBehavior ?? default(FillBehavior);
+			private bool AutoReverse => _owner?.AutoReverse ?? false;
 
 			private T? From => AnimationOwner?.From;
 			private T? To => AnimationOwner?.To;
@@ -94,6 +96,7 @@ public void Begin()
 
 				_activeDuration.Restart();
 				_replayCount = 1;
+				_isReversing = false; // Reset reversing state when beginning
 
 				//Start the animation
 				Play();
@@ -192,7 +195,8 @@ public void SkipToFill()
 				}
 
 				// Set property to its final value
-				var value = ComputeToValue();
+				// With AutoReverse, the final value is the starting value (after reversing back)
+				var value = AutoReverse ? ComputeFromValue() : ComputeToValue();
 				SetValue(value);
 
 				OnEnd();
@@ -225,7 +229,11 @@ private void InitializeAnimator()
 
 				_endValue = ComputeToValue();
 
-				_animator = AnimatorFactory.Create(_owner, _startingValue.Value, _endValue.Value);
+				// If we're in the reverse phase, swap the from and to values
+				var fromValue = _isReversing ? _endValue.Value : _startingValue.Value;
+				var toValue = _isReversing ? _startingValue.Value : _endValue.Value;
+
+				_animator = AnimatorFactory.Create(_owner, fromValue, toValue);
 
 				_animator.SetEasingFunction(this.EasingFunction); //Set the Easing Function of the animator
 
@@ -337,6 +345,22 @@ private void OnEnd()
 			{
 				_animator?.Dispose();
 
+				// Handle AutoReverse: if enabled and we just finished the forward animation, reverse it
+				if (AutoReverse && !_isReversing)
+				{
+					_isReversing = true;
+					// Use Play() instead of Replay() to avoid incrementing _replayCount during the reverse phase.
+					// This ensures RepeatBehavior counts complete cycles (forward + reverse) as single iterations.
+					Play();
+					return;
+				}
+
+				// If we were reversing, we've now completed both forward and reverse
+				if (_isReversing)
+				{
+					_isReversing = false;
+				}
+
 				// If the animation was GPU based, remove the animated value
 
 				if (NeedsRepeat(_activeDuration, _replayCount))

src/Uno.UI/UI/Xaml/Media/Animation/Timeline.cs

@@ -27,6 +27,15 @@ public event EventHandler<object> Completed
 			remove => _completedHandlers?.Remove(value);
 		}
 
+		public bool AutoReverse
+		{
+			get => (bool)GetValue(AutoReverseProperty);
+			set => SetValue(AutoReverseProperty, value);
+		}
+
+		public static DependencyProperty AutoReverseProperty { get; } =
+			DependencyProperty.Register("AutoReverse", typeof(bool), typeof(Timeline), new FrameworkPropertyMetadata(false));
+
 		public Timeline()
 		{
 			IsAutoPropertyInheritanceEnabled = false;