Fix a couple of inheritance bugs.

With associated unit tests.

src/Avalonia.Base/PropertyStore/EffectiveValue`1.cs

@@ -257,11 +257,23 @@ namespace Avalonia.PropertyStore
 
         public void DisposeAndRaiseUnset(ValueStore owner, StyledPropertyBase<T> property)
         {
-            var defaultValue = property.GetDefaultValue(owner.GetType());
+            BindingPriority priority;
+            T oldValue;
 
-            if (!EqualityComparer<T>.Default.Equals(defaultValue, Value))
+            if (property.Inherits && owner.TryGetInheritedValue(property, out var i))
             {
-                owner.Owner.RaisePropertyChanged(property, Value, defaultValue, BindingPriority.Unset, true);
+                oldValue = ((EffectiveValue<T>)i).Value;
+                priority = BindingPriority.Inherited;
+            }
+            else
+            {
+                oldValue = property.GetDefaultValue(owner.GetType());
+                priority = BindingPriority.Unset;
+            }
+
+            if (!EqualityComparer<T>.Default.Equals(oldValue, Value))
+            {
+                owner.Owner.RaisePropertyChanged(property, Value, oldValue, priority, true);
                 if (property.Inherits)
                     owner.OnInheritedEffectiveValueDisposed(property, Value);
             }

src/Avalonia.Base/PropertyStore/ValueStore.cs

@@ -206,6 +206,25 @@ namespace Avalonia.PropertyStore
             return default;
         }
 
+        public bool TryGetInheritedValue(
+            AvaloniaProperty property,
+            [NotNullWhen(true)] out EffectiveValue? result)
+        {
+            Debug.Assert(property.Inherits);
+
+            var i = InheritanceAncestor;
+
+            while (i is not null)
+            {
+                if (i.TryGetEffectiveValue(property, out result))
+                    return true;
+                i = i.InheritanceAncestor;
+            }
+
+            result = null;
+            return false;
+        }
+
         public void SetInheritanceParent(AvaloniaObject? oldParent, AvaloniaObject? newParent)
         {
             var values = DictionaryPool<AvaloniaProperty, OldNewValue>.Get();
@@ -651,7 +670,7 @@ namespace Avalonia.PropertyStore
             if (_effectiveValues is not null && _effectiveValues.Remove(property))
             {
                 if (property.Inherits && --_inheritedValueCount == 0)
-                    OnInheritanceAncestorChanged(InheritanceAncestor?.InheritanceAncestor);
+                    OnInheritanceAncestorChanged(InheritanceAncestor);
                 return true;
             }
 
@@ -663,7 +682,7 @@ namespace Avalonia.PropertyStore
             if (_effectiveValues is not null && _effectiveValues.Remove(property, out result))
             {
                 if (property.Inherits && --_inheritedValueCount == 0)
-                    OnInheritanceAncestorChanged(InheritanceAncestor?.InheritanceAncestor);
+                    OnInheritanceAncestorChanged(InheritanceAncestor);
                 return true;
             }
 
@@ -898,25 +917,6 @@ namespace Avalonia.PropertyStore
             return false;
         }
 
-        private bool TryGetInheritedValue(
-            AvaloniaProperty property,
-            [NotNullWhen(true)] out EffectiveValue? result)
-        {
-            Debug.Assert(property.Inherits);
-
-            var i = InheritanceAncestor;
-
-            while (i is not null)
-            {
-                if (i.TryGetEffectiveValue(property, out result))
-                    return true;
-                i = i.InheritanceAncestor;
-            }
-
-            result = null;
-            return false;
-        }
-
         private EffectiveValue? GetEffectiveValue(AvaloniaProperty property)
         {
             if (_effectiveValues is not null && _effectiveValues.TryGetValue(property, out var value))

tests/Avalonia.Base.UnitTests/AvaloniaObjectTests_Inheritance.cs

@@ -43,6 +43,53 @@ namespace Avalonia.Base.UnitTests
             Assert.Equal("bazdefault", child.GetValue(Class1.BazProperty));
         }
 
+        [Fact]
+        public void ClearValue_On_Parent_Raises_PropertyChanged_On_Child()
+        {
+            Class1 parent = new Class1();
+            Class2 child = new Class2 { Parent = parent };
+            var raised = 0;
+
+            parent.SetValue(Class1.BazProperty, "changed");
+
+            child.PropertyChanged += (s, e) =>
+            {
+                Assert.Same(child, e.Sender);
+                Assert.Equal("changed", e.OldValue);
+                Assert.Equal("bazdefault", e.NewValue);
+                Assert.Equal(BindingPriority.Inherited, e.Priority);
+                ++raised;
+            };
+
+            parent.ClearValue(Class1.BazProperty);
+
+            Assert.Equal(1, raised);
+        }
+
+        [Fact]
+        public void ClearValue_On_Child_Raises_PropertyChanged_With_Inherited_Parent_Value()
+        {
+            Class1 parent = new Class1();
+            Class2 child = new Class2 { Parent = parent };
+            var raised = 0;
+
+            parent.SetValue(Class1.BazProperty, "parent");
+            child.SetValue(Class1.BazProperty, "child");
+
+            child.PropertyChanged += (s, e) =>
+            {
+                Assert.Same(child, e.Sender);
+                Assert.Equal("child", e.OldValue);
+                Assert.Equal("parent", e.NewValue);
+                Assert.Equal(BindingPriority.Inherited, e.Priority);
+                ++raised;
+            };
+
+            child.ClearValue(Class1.BazProperty);
+
+            Assert.Equal(1, raised);
+        }
+
         [Fact]
         public void Setting_InheritanceParent_Raises_PropertyChanged_When_Parent_Has_Value_Set()
         {

tests/Avalonia.Base.UnitTests/PropertyStore/ValueStoreTests_Inheritance.cs

@@ -80,6 +80,23 @@ namespace Avalonia.Base.UnitTests.PropertyStore
             Assert.Same(child.GetValueStore(), grandchild.GetValueStore().InheritanceAncestor);
         }
 
+        [Fact]
+        public void Clearing_Value_In_Child_Updates_InheritanceAncestor()
+        {
+            var parent = new Class1();
+            var child = new Class1 { Parent = parent };
+            var grandchild = new Class1 { Parent = child };
+
+            parent.Foo = "changed";
+            child.Foo = "foochanged";
+            child.ClearValue(Class1.FooProperty);
+
+            var parentStore = parent.GetValueStore();
+            Assert.Null(parentStore.InheritanceAncestor);
+            Assert.Same(parentStore, child.GetValueStore().InheritanceAncestor);
+            Assert.Same(parentStore, grandchild.GetValueStore().InheritanceAncestor);
+        }
+
         [Fact]
         public void Adding_Child_Sets_InheritanceAncestor()
         {