I finally figured this out right after posting the question

replacing the

 return RedirectToLocal(returnUrl);

with this line of code

return Content("<script> if(window.opener) {window.opener.location.reload(); window.close();} else{window.location = '" + Url.Action("Index", "Home") + "';} </script>", "text/html");

so passing back javascript to check for the popup, which will close it, and also refresh the page.