There is no perfect backwards compatible solution out there afaik, but this is getting me pretty close to where I want it to be.

• instead of redirecting set a empty 278 response (made up status code) for xhr requests
• tell jquery to redirect when receiving a 278 response
• the success handlers are still executed / there is no way to stop them, but they can either just insert the blank reply or do a != '' check

// application.js
// when we get a fake redirect response, go there
// this will still execute success handlers, but hopefully the fail or are not ugly ...
$.ajaxSetup({
  statusCode: {
    278: function(_,_,response){
      // using turbolinks to not lose back functionality
      Turbolinks.visit(response.getResponseHeader('X-278-redirect'));
    }
  }
})

# some controller
redirect_to_with_xhr signup_path

# application_controller.rb
# ajax requests follow all redirects, so we have to improvise with a
# special code and header to not get placeholders replaced with full pages.
def redirect_to_with_xhr(path)
  if request.xhr?
    response.headers["X-278-redirect"] = path
    head 278
  else
    redirect_to path
  end
end