Add unit test for FillMode

tests/Avalonia.Animation.UnitTests/AnimationIterationTests.cs

@@ -8,6 +8,7 @@ using Avalonia.Styling;
 using Avalonia.UnitTests;
 using Avalonia.Data;
 using Xunit;
+using Avalonia.Animation.Easings;
 
 namespace Avalonia.Animation.UnitTests
 {
@@ -73,5 +74,58 @@ namespace Avalonia.Animation.UnitTests
             Assert.True(animationRun.Status == TaskStatus.RanToCompletion);
             Assert.Equal(border.Width, 100d);
         }
+
+        [Fact]
+        public void Check_FillModes_Start_and_End_Values_if_Retained()
+        {
+            var keyframe1 = new KeyFrame()
+            {
+                Setters =
+                {
+                    new Setter(Border.WidthProperty, 0d),
+                },
+                Cue = new Cue(0.0d)
+            };
+
+            var keyframe2 = new KeyFrame()
+            {
+                Setters =
+                {
+                    new Setter(Border.WidthProperty, 300d),
+                },
+                Cue = new Cue(1.0d)
+            };
+
+            var animation = new Animation()
+            {
+                Duration = TimeSpan.FromSeconds(0.05d),
+                Delay = TimeSpan.FromSeconds(0.05d),
+                Easing = new SineEaseInOut(),
+                FillMode = FillMode.Both,
+                Children =
+                {
+                    keyframe1,
+                    keyframe2
+                }
+            };
+
+            var border = new Border()
+            {
+                Height = 100d,
+                Width = 100d,
+            };
+
+            var clock = new TestClock();
+            var animationRun = animation.RunAsync(border, clock);
+
+            clock.Step(TimeSpan.FromSeconds(0d));
+            Assert.Equal(border.Width, 0d);
+
+            clock.Step(TimeSpan.FromSeconds(0.050d));
+            Assert.Equal(border.Width, 0d);
+
+            clock.Step(TimeSpan.FromSeconds(0.100d));
+            Assert.Equal(border.Width, 300d);
+        }
     }
 }