Add transitions unit test.

src/Avalonia.Animation/TransitionInstance.cs

@@ -31,7 +31,7 @@ namespace Avalonia.Animation
             var interpVal = (double)t.Ticks / _duration.Ticks;
 
             // Clamp interpolation value.
-            if (interpVal >= 1d | (interpVal < 0d))
+            if (interpVal >= 1d | interpVal < 0d)
             {
                 PublishNext(1d);
                 PublishCompleted();

tests/Avalonia.Animation.UnitTests/AnimationTransitionsTest.cs

@@ -0,0 +1,73 @@
+using System;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Avalonia.Animation;
+using Avalonia.Controls;
+using Avalonia.Styling;
+using Avalonia.UnitTests;
+using Avalonia.Data;
+using Xunit;
+using Avalonia.Animation.Easings;
+
+namespace Avalonia.Animation.UnitTests
+{
+    public class AnimationTransitionsTests
+    {
+        [Fact]
+        public void Check_Transitions_Interpolation_Negative_Bounds_Clamp()
+        {
+            var clock = new MockGlobalClock();
+
+            using (UnitTestApplication.Start(new TestServices(globalClock: clock)))
+            {
+                var border = new Border
+                {
+                    Transitions =
+                    {
+                        new DoubleTransition
+                        {
+                            Duration = TimeSpan.FromSeconds(1),
+                            Property = Border.OpacityProperty,
+                        }
+                    }
+                };
+
+                border.Opacity = 0;
+
+                clock.Pulse(TimeSpan.FromSeconds(0));
+                clock.Pulse(TimeSpan.FromSeconds(-0.5));
+
+                Assert.Equal(0, border.Opacity);
+            }
+        }
+
+        [Fact]
+        public void Check_Transitions_Interpolation_Positive_Bounds_Clamp()
+        {
+            var clock = new MockGlobalClock();
+
+            using (UnitTestApplication.Start(new TestServices(globalClock: clock)))
+            {
+                var border = new Border
+                {
+                    Transitions =
+                    {
+                        new DoubleTransition
+                        {
+                            Duration = TimeSpan.FromSeconds(1),
+                            Property = Border.OpacityProperty,
+                        }
+                    }
+                };
+
+                border.Opacity = 0;
+
+                clock.Pulse(TimeSpan.FromSeconds(0));
+                clock.Pulse(TimeSpan.FromMilliseconds(1001));
+
+                Assert.Equal(0, border.Opacity);
+            }
+        }
+    }
+}