anchor.diff:  open System.Windows.Forms.Integration
   
   type RenderControl() =
  -    inherit ContentControl()
  +    inherit Border()
   
       let mutable runtime : IRuntime = Unchecked.defaultof<IRuntime>
       let mutable renderTask : Option<IRenderTask> = None
  
positive.code:     override x.OnRenderSizeChanged(e) =
          base.OnRenderSizeChanged(e)
          transact (fun () -> Mod.change sizes (V2i(x.ActualWidth, x.ActualHeight)))
  
      member x.Implementation
          with get() = match ctrl with | Some c -> c | _ -> null
  
negative.code:             | Some i -> failwith "implementation can only be set once per control"
              | None -> ()
  
          self.Child <- c
          runtime <- cr.Runtime
  
          match c with
  