Adding other components to LoginForm Panel doesn't work

I have a Panel subclass where I try to add a LoginForm, and two other components. When I add just the LoginForm alone, it shows up. When I try to add two additional components, nothing appears on the screen:

    _loginForm = new LoginForm();
    getContent().addComponent(_loginForm);

    _progress = new ProgressIndicator();
    _progress.setIndeterminate(false);
    _progress.setPollingInterval(0);
    _progress.setValue(0);
    getContent().addComponent(_progress);

    _field = new TextField();
    _field.setReadOnly(true);
    getContent().addComponent(_field);

I am also adding a Refresher on the page in the attach() method, like this

    _refresher = new Refresher();
    _refresher.setRefreshInterval(250);
    _refresher.setEnabled(true);
    getWindow().addComponent(_refresher);

Does anyone know why things stop working when I add the ProgressIndicator and TextField to the Panel?

I’ve noticed that I can add the TextField with no problem, but as soon as I add the ProgressIndicator, the UI disappears.

OK, I figured it out. Adding both a Refresher and ProgressIndicator to the same window doesn’t work. You have to add one or the other. Can anyone explain exactly why?