Eternal 'red ring of death' with slow Servlets

(related to this post)

In my application I open a URL using 'Window.open(new ExternalResource(url))'. Behind the url is a servlet (I use embedded Jetty), with following code:

response.setContentType("plain/text");
            response.setHeader("Content-Disposition", "attachment; fileName="test.txt");

            ServletOutputStream out = response.getOutputStream();
            out.println("header");
            out.flush();

            response.setStatus(HttpServletResponse.SC_OK);

(response being a HttpServletResponse)

And this works perfectly. However, if I add a 'sleep' to this servlet so that the request takes long enough to start the spinning '*' in the top-right corner of my application, this spinning '*' just stays there, even after the data associated with the URL is downloaded.

So, if I change the servlet to:

response.setContentType("plain/text");
            response.setHeader("Content-Disposition", "attachment; fileName="test.txt");
            
            Thread.sleep(5000);

            ServletOutputStream out = response.getOutputStream();
            out.println("header");
            out.flush();

            response.setStatus(HttpServletResponse.SC_OK);

I get the same content that is downloaded, but my application doesn't do anything anymore, except displaying a red spinning '*'.

Why the sleep? If you look at the related post (this one), it turned out that the request in this particular case took quite some time to complete, and it always resulted in a spinning '*'. I can only reproduce this on Chrome (I'm using version 18.0.1025.168, but it's aslo the case with older versions).

Any ideas on how to solve this?