More tests forContentPresenter and InheritanceParent.

Make sure the inheritance parent is reset for non-rooted and standalone `ContentPresenter`s.

src/Avalonia.Controls/Presenters/ContentPresenter.cs

@@ -237,7 +237,7 @@ namespace Avalonia.Controls.Presenters
                     // template.
                     LogicalChildren.Remove(oldChild);
                 }
-                else
+                else if (TemplatedParent != null)
                 {
                     // If we're in a ContentControl's template then invoke ChildChanging to let
                     // ContentControlMixin handle removing the logical child.
@@ -248,6 +248,10 @@ namespace Avalonia.Controls.Presenters
                         newChild,
                         BindingPriority.LocalValue));
                 }
+                else if (oldChild != null)
+                {
+                    ((ISetInheritanceParent)oldChild).SetParent(oldChild.Parent);
+                }
             }
 
             // Set the DataContext if the data isn't a control.
@@ -433,6 +437,7 @@ namespace Avalonia.Controls.Presenters
             {
                 VisualChildren.Remove(Child);
                 LogicalChildren.Remove(Child);
+                ((ISetInheritanceParent)Child).SetParent(Child.Parent);
                 Child = null;
                 _dataTemplate = null;
             }

tests/Avalonia.Controls.UnitTests/Presenters/ContentPresenterTests_Standalone.cs

@@ -251,5 +251,21 @@ namespace Avalonia.Controls.UnitTests.Presenters
 
             target.Content = 42;
         }
+
+        [Fact]
+        public void Should_Reset_InheritanceParent_When_Child_Removed()
+        {
+            var logicalParent = new Canvas();
+            var child = new TextBlock();
+            var target = new ContentPresenter();
+            var root = new TestRoot(target);
+
+            ((ISetLogicalParent)child).SetParent(logicalParent);
+            target.Content = child;
+            target.Content = null;
+
+            // InheritanceParent is exposed via StylingParent.
+            Assert.Same(logicalParent, ((IStyledElement)child).StylingParent);
+        }
     }
 }

tests/Avalonia.Controls.UnitTests/Presenters/ContentPresenterTests_Unrooted.cs

@@ -98,5 +98,22 @@ namespace Avalonia.Controls.UnitTests.Presenters
             target.ApplyTemplate();
             Assert.IsType<Decorator>(target.Child);
         }
+
+        [Fact]
+        public void Should_Reset_InheritanceParent_When_Child_Removed()
+        {
+            var logicalParent = new Canvas();
+            var child = new TextBlock();
+            var target = new ContentPresenter();
+
+            ((ISetLogicalParent)child).SetParent(logicalParent);
+            target.Content = child;
+            target.UpdateChild();
+            target.Content = null;
+            target.UpdateChild();
+
+            // InheritanceParent is exposed via StylingParent.
+            Assert.Same(logicalParent, ((IStyledElement)child).StylingParent);
+        }
     }
 }