Correctly remove ContentPresenter's content from its parent host

When the content is updated while being detached from the visual tree.

src/Avalonia.Controls/Presenters/ContentPresenter.cs

@@ -1,5 +1,5 @@
 using System;
-
+using Avalonia.Collections;
 using Avalonia.Controls.Documents;
 using Avalonia.Controls.Metadata;
 using Avalonia.Controls.Primitives;
@@ -442,7 +442,7 @@ namespace Avalonia.Controls.Presenters
             var contentTemplate = ContentTemplate;
             var oldChild = Child;
             var newChild = CreateChild(content, oldChild, contentTemplate);
-            var logicalChildren = Host?.LogicalChildren ?? LogicalChildren;
+            var logicalChildren = GetEffectiveLogicalChildren();
 
             // Remove the old child if we're not recycling it.
             if (newChild != oldChild)
@@ -488,6 +488,9 @@ namespace Avalonia.Controls.Presenters
 
         }
 
+        private IAvaloniaList<ILogical> GetEffectiveLogicalChildren()
+            => Host?.LogicalChildren ?? LogicalChildren;
+
         /// <inheritdoc/>
         protected override void OnAttachedToLogicalTree(LogicalTreeAttachmentEventArgs e)
         {
@@ -692,7 +695,7 @@ namespace Avalonia.Controls.Presenters
             else if (Child != null)
             {
                 VisualChildren.Remove(Child);
-                LogicalChildren.Remove(Child);
+                GetEffectiveLogicalChildren().Remove(Child);
                 ((ISetInheritanceParent)Child).SetParent(Child.Parent);
                 Child = null;
                 _recyclingDataTemplate = null;

tests/Avalonia.Controls.UnitTests/ContentControlTests.cs

@@ -359,16 +359,21 @@ namespace Avalonia.Controls.UnitTests
             target.Presenter.ApplyTemplate();
 
             Assert.Equal(target, target.Presenter.Child.GetLogicalParent());
+            Assert.Equal(new[] { target.Presenter.Child }, target.LogicalChildren);
 
             root.Child = null;
 
             Assert.Null(target.Template);
 
             target.Content = null;
+
+            Assert.Empty(target.LogicalChildren);
+
             root.Child = target;
             target.Content = "Bar";
 
             Assert.Equal(target, target.Presenter.Child.GetLogicalParent());
+            Assert.Equal(new[] { target.Presenter.Child }, target.LogicalChildren);
         }
 
         private static FuncControlTemplate GetTemplate()