Raised property changed when binding completes.

Fixes #3545

src/Avalonia.Base/AvaloniaObject.cs

@@ -478,7 +478,13 @@ namespace Avalonia
             }
         }
 
-        void IValueSink.Completed(AvaloniaProperty property, IPriorityValueEntry entry) { }
+        void IValueSink.Completed<T>(
+            StyledPropertyBase<T> property,
+            IPriorityValueEntry entry,
+            Optional<T> oldValue) 
+        {
+            ((IValueSink)this).ValueChanged(property, BindingPriority.Unset, oldValue, default);
+        }
 
         /// <summary>
         /// Called for each inherited property when the <see cref="InheritanceParent"/> changes.

src/Avalonia.Base/PropertyStore/BindingEntry.cs

@@ -48,10 +48,10 @@ namespace Avalonia.PropertyStore
         {
             _subscription?.Dispose();
             _subscription = null;
-            _sink.Completed(Property, this);
+            _sink.Completed(Property, this, Value);
         }
 
-        public void OnCompleted() => _sink.Completed(Property, this);
+        public void OnCompleted() => _sink.Completed(Property, this, Value);
 
         public void OnError(Exception error)
         {

src/Avalonia.Base/PropertyStore/IValueSink.cs

@@ -15,6 +15,9 @@ namespace Avalonia.PropertyStore
             Optional<T> oldValue,
             BindingValue<T> newValue);
 
-        void Completed(AvaloniaProperty property, IPriorityValueEntry entry);
+        void Completed<T>(
+            StyledPropertyBase<T> property,
+            IPriorityValueEntry entry,
+            Optional<T> oldValue);
     }
 }

src/Avalonia.Base/PropertyStore/PriorityValue.cs

@@ -117,7 +117,10 @@ namespace Avalonia.PropertyStore
             UpdateEffectiveValue();
         }
 
-        void IValueSink.Completed(AvaloniaProperty property, IPriorityValueEntry entry)
+        void IValueSink.Completed<TValue>(
+            StyledPropertyBase<TValue> property,
+            IPriorityValueEntry entry,
+            Optional<TValue> oldValue)
         {
             _entries.Remove((IPriorityValueEntry<T>)entry);
             UpdateEffectiveValue();

src/Avalonia.Base/ValueStore.cs

@@ -190,13 +190,17 @@ namespace Avalonia
             _sink.ValueChanged(property, priority, oldValue, newValue);
         }
 
-        void IValueSink.Completed(AvaloniaProperty property, IPriorityValueEntry entry)
+        void IValueSink.Completed<T>(
+            StyledPropertyBase<T> property,
+            IPriorityValueEntry entry,
+            Optional<T> oldValue)
         {
             if (_values.TryGetValue(property, out var slot))
             {
                 if (slot == entry)
                 {
                     _values.Remove(property);
+                    _sink.Completed(property, entry, oldValue);
                 }
             }
         }