Layout integration seems to be working now

src/Avalonia.Layout/LayoutManager.cs

@@ -151,7 +151,7 @@ namespace Avalonia.Layout
 
             if (root != null)
             {
-                root.Arrange(new Rect(root.DesiredSize));
+                root.Arrange(new Rect(root.ClientSize));
             }
             else if (parent != null)
             {

src/Avalonia.Layout/Layoutable.cs

@@ -399,7 +399,7 @@ namespace Avalonia.Layout
         }
 
         /// <inheritdoc/>
-        public virtual void ChildDesiredSizeChanged(ILayoutable control)
+        void ILayoutable.ChildDesiredSizeChanged(ILayoutable control)
         {
             if (!_measuring)
             {

src/Windows/Avalonia.Win32.Interop/Wpf/WpfTopLevelImpl.cs

@@ -40,10 +40,10 @@ namespace Avalonia.Win32.Interop.Wpf
                 EnforceClientSize = false;
             }
 
-            public override void ChildDesiredSizeChanged(ILayoutable control)
+            public override void InvalidateMeasure()
             {
                 ((FrameworkElement)PlatformImpl)?.InvalidateMeasure();
-                base.ChildDesiredSizeChanged(control);
+                base.InvalidateMeasure();
             }
 
             protected override void HandleResized(Size clientSize)
@@ -52,12 +52,6 @@ namespace Avalonia.Win32.Interop.Wpf
                 LayoutManager.Instance.ExecuteLayoutPass();
                 Renderer?.Resized(clientSize);
             }
-
-            protected override void ArrangeCore(Rect finalRect)
-            {
-                base.ArrangeOverride(finalRect.Size);
-                Bounds = finalRect;
-            }
         }
 
         public WpfTopLevelImpl()