Implement @maxkatz6's suggestions

src/Avalonia.Base/Rendering/Composition/CompositionExternalMemory.cs

@@ -6,6 +6,8 @@ using Avalonia.Metadata;
 using Avalonia.Platform;
 
 namespace Avalonia.Rendering.Composition;
+
+[NotClientImplementable]
 public interface ICompositionGpuInterop
 {
     /// <summary>
@@ -99,7 +101,7 @@ public interface ICompositionGpuImportedObject : IAsyncDisposable
     /// <seealso cref="ImportCompleted">ImportCompleted (recommended replacement)</seealso>
     [Obsolete("Please use ICompositionGpuImportedObject.ImportCompleted instead")]
     [EditorBrowsable(EditorBrowsableState.Never)]
-    Task ImportCompeted => ImportCompleted;
+    Task ImportCompeted { get; }
     
     /// <summary>
     /// Indicates if the device context this instance is associated with is no longer available

src/Avalonia.Base/Rendering/Composition/CompositionInterop.cs

@@ -74,6 +74,8 @@ abstract class CompositionGpuImportedObjectBase : ICompositionGpuImportedObject
     public abstract void Dispose();
 
     public Task ImportCompleted { get; }
+
+    public Task ImportCompeted => ImportCompleted;
     public bool IsLost => Context.IsLost;
 
     public ValueTask DisposeAsync() => new(Compositor.InvokeServerJobAsync(() =>