Index: System.Windows.Forms/XplatUIWin32.cs
===================================================================
--- System.Windows.Forms/XplatUIWin32.cs        (revision 72565)
+++ System.Windows.Forms/XplatUIWin32.cs        (working copy)
@@ -1201,6 +1201,8 @@
                                Win32MessageBox(IntPtr.Zero, "Error : " + error.ToString(), "Failed to create window, class '"+cp.ClassName+"'", 0);
                        }

+                       hwnd.ClientWindow = WindowHandle;
+
                        Win32SetWindowLong(WindowHandle, WindowLong.GWL_USERDATA, (uint)ThemeEngine.Current.DefaultControlBackColor.ToArgb());

                        return WindowHandle;