Added another failing transitions test.

tests/Avalonia.Animation.UnitTests/AnimatableTests.cs

@@ -339,6 +339,28 @@ namespace Avalonia.Animation.UnitTests
             control.Transitions = new Transitions { target.Object };
         }
 
+        [Fact]
+        public void Transitions_Can_Re_Set_During_Batch_Update()
+        {
+            var target = CreateTarget();
+            var control = CreateControl(target.Object);
+
+            // Assigning and then clearing Transitions ensures we have a transition state
+            // collection created.
+            control.Transitions = null;
+
+            control.BeginBatchUpdate();
+
+            // Setting opacity then Transitions means that we receive the Transitions change
+            // after the Opacity change when EndBatchUpdate is called.
+            control.Opacity = 0.5;
+            control.Transitions = new Transitions { target.Object };
+
+            // Which means that the transition state hasn't been initialized with the new
+            // Transitions when the Opacity change notification gets raised here.
+            control.EndBatchUpdate();
+        }
+
         private static Mock<ITransition> CreateTarget()
         {
             return CreateTransition(Visual.OpacityProperty);