fix: Align Duration behavior with WinUI

src/Uno.UI.Tests/Animations/Given_Duration.cs

@@ -84,5 +84,78 @@ namespace Uno.UI.Tests.Animations
 			Assert.AreEqual(new Duration(TimeSpan.FromSeconds(48)), duration.Add(new Duration(TimeSpan.FromSeconds(24))));
 			Assert.AreEqual(new Duration(TimeSpan.FromSeconds(1)), duration.Subtract(new Duration(TimeSpan.FromSeconds(23))));
 		}
+
+		[TestMethod]
+		[DataRow("00:00:05", "00:00:03", "00:00:08")]
+		[DataRow("00:00:05", "Automatic", "Automatic")]
+		[DataRow("00:00:05", "Forever", "Forever")]
+		[DataRow("Automatic", "00:00:03", "Automatic")]
+		[DataRow("Automatic", "Automatic", "Automatic")]
+		[DataRow("Automatic", "Forever", "Automatic")]
+		[DataRow("Forever", "00:00:03", "Forever")]
+		[DataRow("Forever", "Automatic", "Automatic")]
+		[DataRow("Forever", "Forever", "Forever")]
+		public void When_Adding_Durations(string d1, string d2, string expected)
+		{
+			var duration1 = StringToDuration(d1);
+			var duration2 = StringToDuration(d2);
+			var expectedDuration = StringToDuration(expected);
+			Assert.AreEqual(expectedDuration, duration1 + duration2);
+			Assert.AreEqual(expectedDuration, duration1.Add(duration2));
+			Assert.AreEqual(expectedDuration, duration2.Add(duration1));
+		}
+
+		[TestMethod]
+		[DataRow("00:00:05", "00:00:03", "00:00:02")]
+		[DataRow("00:00:05", "Automatic", "Automatic")]
+		[DataRow("00:00:05", "Forever", "Automatic")]
+		[DataRow("Automatic", "00:00:03", "Automatic")]
+		[DataRow("Automatic", "Automatic", "Automatic")]
+		[DataRow("Automatic", "Forever", "Automatic")]
+		[DataRow("Forever", "00:00:03", "Forever")]
+		[DataRow("Forever", "Automatic", "Automatic")]
+		[DataRow("Forever", "Forever", "Automatic")]
+		public void When_Subtracting_Durations(string d1, string d2, string expected)
+		{
+			var duration1 = StringToDuration(d1);
+			var duration2 = StringToDuration(d2);
+			var expectedDuration = StringToDuration(expected);
+			Assert.AreEqual(expectedDuration, duration1 - duration2);
+			Assert.AreEqual(expectedDuration, duration1.Subtract(duration2));
+		}
+
+		[TestMethod]
+		[DataRow("00:00:05", "00:00:03", 1, true, true, false, false)]
+		[DataRow("00:00:03", "00:00:05", -1, false, false, true, true)]
+		[DataRow("00:00:03", "00:00:03", 0, false, true, false, true)]
+		[DataRow("00:00:05", "Automatic", 1, false, false, false, false)]
+		[DataRow("00:00:05", "Forever", -1, false, false, true, true)]
+		[DataRow("Automatic", "00:00:03", -1, false, false, false, false)]
+		[DataRow("Automatic", "Automatic", 0, false, true, false, true)]
+		[DataRow("Automatic", "Forever", -1, false, false, false, false)]
+		[DataRow("Forever", "00:00:03", 1, true, true, false, false)]
+		[DataRow("Forever", "Automatic", 1, false, false, false, false)]
+		[DataRow("Forever", "Forever", 0, false, true, false, true)]
+		public void When_Comparing_Durations(string d1, string d2, int expectedCompare, bool isGreaterThan, bool isGreaterThanOrEquals, bool isLessThan, bool isLessThanOrEquals)
+		{
+			var duration1 = StringToDuration(d1);
+			var duration2 = StringToDuration(d2);
+			Assert.AreEqual(expectedCompare, duration1.CompareTo(duration2));
+			Assert.AreEqual(expectedCompare, Duration.Compare(duration1, duration2));
+			Assert.AreEqual(isGreaterThan, duration1 > duration2);
+			Assert.AreEqual(isGreaterThanOrEquals, duration1 >= duration2);
+			Assert.AreEqual(isLessThan, duration1 < duration2);
+			Assert.AreEqual(isLessThanOrEquals, duration1 <= duration2);
+		}
+
+		private Duration StringToDuration(string s)
+		{
+			return s switch
+			{
+				"Automatic" => Duration.Automatic,
+				"Forever" => Duration.Forever,
+				_ => new Duration(TimeSpan.Parse(s)),
+			};
+		}
 	}
 }

src/Uno.UI/UI/Xaml/Duration.cs

 using Uno.UI.Extensions;
 using System;
-using System.Collections.Generic;
 using System.Globalization;
-using System.Text;
 
 namespace Windows.UI.Xaml
 {
@@ -11,7 +9,6 @@ namespace Windows.UI.Xaml
 		private static readonly string __automatic = "Automatic";
 		private static readonly string __forever = "Forever";
 
-
 		public Duration(TimeSpan timeSpan)
 		{
 			Type = DurationType.TimeSpan;
@@ -24,48 +21,67 @@ namespace Windows.UI.Xaml
 		public static implicit operator Duration(string timeSpan)
 			=> timeSpan != null ? new Duration(TimeSpan.Parse(timeSpan, CultureInfo.InvariantCulture)) : new Duration(TimeSpan.Zero);
 
-		public bool HasTimeSpan
-		{
-			get
-			{
-				return this.Type == DurationType.TimeSpan;
-			}
-		}
+		public bool HasTimeSpan => Type == DurationType.TimeSpan;
 
-		public static Duration Forever
-		{
-			get
-			{
-				return new Duration() { Type = DurationType.Forever };
-			}
-		}
+		public static Duration Forever => new Duration() { Type = DurationType.Forever };
 
-		public static Duration Automatic
-		{
-			get
-			{
-				return new Duration() { Type = DurationType.Automatic };
-			}
-		}
+		public static Duration Automatic => new Duration() { Type = DurationType.Automatic };
 
 		public Duration Add(Duration duration)
 		{
+			// We have 9 cases:
+			// (1) TimeSpan + TimeSpan
+			// (2) TimeSpan + Automatic
+			// (3) TimeSpan + Forever
+			// (4) Automatic + TimeSpan
+			// (5) Automatic + Automatic
+			// (6) Automatic + Forever
+			// (7) Forever + TimeSpan
+			// (8) Forever + Automatic
+			// (9) Forever + Forever
+
+			// Case (1)
 			if (this.Type == DurationType.TimeSpan && duration.Type == DurationType.TimeSpan)
 			{
 				return new Duration(this.TimeSpan.Add(duration.TimeSpan));
 			}
 
-			return this;
+			// Case (2), (4), (5), (6), (8)
+			if (this.Type == DurationType.Automatic || duration.Type == DurationType.Automatic)
+			{
+				return Automatic;
+			}
+
+			// Case (3), (7), (9)
+			return Forever;
 		}
 
 		public Duration Subtract(Duration duration)
 		{
+			// We have 9 cases:
+			// (1) TimeSpan - TimeSpan     ===> Subtract spans
+			// (2) TimeSpan - Automatic    ===> Automatic
+			// (3) TimeSpan - Forever	   ===> Automatic
+			// (4) Automatic - TimeSpan    ===> Automatic
+			// (5) Automatic - Automatic   ===> Automatic
+			// (6) Automatic - Forever	   ===> Automatic
+			// (7) Forever - TimeSpan      ===> Forever
+			// (8) Forever - Automatic     ===> Automatic
+			// (9) Forever - Forever       ===> Automatic
+
+			// Case (1)
 			if (this.Type == DurationType.TimeSpan && duration.Type == DurationType.TimeSpan)
 			{
 				return new Duration(this.TimeSpan.Subtract(duration.TimeSpan));
 			}
 
-			return this;
+			// Case (7)
+			if (this.Type == DurationType.Forever && duration.Type == DurationType.TimeSpan)
+			{
+				return Forever;
+			}
+
+			return Automatic;
 		}
 
 		#region Operator overrides
@@ -82,22 +98,67 @@ namespace Windows.UI.Xaml
 
 		public static bool operator >(Duration t1, Duration t2)
 		{
-			return Compare(t1, t2) > 0;
+			if (t1.HasTimeSpan && t2.HasTimeSpan)
+			{
+				return t1.TimeSpan > t2.TimeSpan;
+			}
+
+			if (t1.HasTimeSpan && t2.Type == DurationType.Forever)
+			{
+				return false;
+			}
+
+			if (t1.Type == DurationType.Forever && t2.HasTimeSpan)
+			{
+				return true;
+			}
+
+			return false;
 		}
 
 		public static bool operator >=(Duration t1, Duration t2)
 		{
-			return Compare(t1, t2) >= 0;
+			if (t1.Type == DurationType.Automatic && t2.Type == DurationType.Automatic)
+			{
+				return true;
+			}
+
+			if (t1.Type == DurationType.Automatic || t2.Type == DurationType.Automatic)
+			{
+				return false;
+			}
+
+			return !(t1 < t2);
 		}
 
 		public static bool operator <(Duration t1, Duration t2)
 		{
-			return Compare(t1, t2) < 0;
+			if (t1.HasTimeSpan && t2.HasTimeSpan)
+			{
+				return t1.TimeSpan < t2.TimeSpan;
+			}
+
+			if (t1.HasTimeSpan && t2.Type == DurationType.Forever)
+			{
+				return true;
+			}
+
+			return false;
 		}
 
 		public static bool operator <=(Duration t1, Duration t2)
 		{
-			return Compare(t1, t2) <= 0;
+			if (t1.Type == DurationType.Automatic && t2.Type == DurationType.Automatic)
+			{
+				return true;
+			}
+
+			if (t1.Type == DurationType.Automatic || t2.Type == DurationType.Automatic)
+			{
+				return false;
+			}
+
+			return !(t1 > t2);
 		}
 
 		public static Duration operator +(Duration t1, Duration t2)