From edeca6a4db9e5e7b1c77c8013a241b85ea6c332f Mon Sep 17 00:00:00 2001
From: Tom Edwards <109803929+TomEdwardsEnscape@users.noreply.github.com>
Date: Wed, 28 May 2025 01:29:50 +0200
Subject: [PATCH] Fixed transitions with delay but no duration completing
 instantly (#18929)

---
 .../Animation/TransitionInstance.cs           | 12 +++++-
 .../Animation/TransitionsTests.cs             | 43 ++++++++++++++++++-
 2 files changed, 51 insertions(+), 4 deletions(-)

diff --git a/src/Avalonia.Base/Animation/TransitionInstance.cs b/src/Avalonia.Base/Animation/TransitionInstance.cs
index 9c9494ff87..676a30e81e 100644
--- a/src/Avalonia.Base/Animation/TransitionInstance.cs
+++ b/src/Avalonia.Base/Animation/TransitionInstance.cs
@@ -33,9 +33,17 @@ namespace Avalonia.Animation
             //                   ^- normalizedDelayEnd
             //                    [<----   normalizedInterpVal   --->]
 
-            var normalizedInterpVal = 1d;
+            double normalizedInterpVal;
 
-            if (!MathUtilities.AreClose(_duration.TotalSeconds, 0d))
+            if (t < _delay)
+            {
+                normalizedInterpVal = 0d;
+            }
+            else if (MathUtilities.AreClose(_duration.TotalSeconds, 0d))
+            {
+                normalizedInterpVal = 1d;
+            }
+            else
             {
                 var normalizedTotalDur = _delay + _duration;
                 var normalizedDelayEnd = _delay.TotalSeconds / normalizedTotalDur.TotalSeconds;
diff --git a/tests/Avalonia.Base.UnitTests/Animation/TransitionsTests.cs b/tests/Avalonia.Base.UnitTests/Animation/TransitionsTests.cs
index 2737c2cebf..caaa29d05f 100644
--- a/tests/Avalonia.Base.UnitTests/Animation/TransitionsTests.cs
+++ b/tests/Avalonia.Base.UnitTests/Animation/TransitionsTests.cs
@@ -84,7 +84,8 @@ namespace Avalonia.Base.UnitTests.Animation
             var clock = new TestClock();
 
             var i = -1;
-            
+            var completed = false;
+
             new TransitionInstance(clock, TimeSpan.FromMilliseconds(30), TimeSpan.FromMilliseconds(70)).Subscribe(
                 nextValue =>
                 {
@@ -124,12 +125,50 @@ namespace Avalonia.Base.UnitTests.Animation
                             Assert.Equal(1d, nextValue);
                             break;
                     }
-                });
+                }, () => completed = true);
 
             for (var z = 0; z <= 10; z++)
             {
                 clock.Pulse(TimeSpan.FromMilliseconds(10));
             }
+
+            Assert.True(completed);
+        }
+
+        [Fact]
+        public void TransitionInstance_With_Delay_But_Zero_Duration_Is_Completed_After_Delay()
+        {
+            var clock = new TestClock();
+
+            var i = -1;
+            var completed = false;
+
+            new TransitionInstance(clock, TimeSpan.FromMilliseconds(30), TimeSpan.Zero).Subscribe(
+                nextValue =>
+                {
+                    switch (i++)
+                    {
+                        case 0:
+                            Assert.Equal(0, nextValue);
+                            break;
+                        case 1:
+                            Assert.Equal(0, nextValue);
+                            break;
+                        case 2:
+                            Assert.Equal(0, nextValue);
+                            break;
+                        case 3: // one iteration sooner than the test above, because the start of the transition is also the end
+                            Assert.Equal(1, nextValue);
+                            break;
+                    }
+                }, () => completed = true);
+
+            for (var z = 0; z <= 4; z++)
+            {
+                clock.Pulse(TimeSpan.FromMilliseconds(10));
+            }
+
+            Assert.True(completed);
         }
     }
 }