When dismissing a Javascript dialog via the close button the webpage which showed it is not responsive. With the quicknanobrowser example go to:

http://www.javascripter.net/faq/prompt.htm

and then click on Try It Now. Close the resulting dialog via the X and then try to interact with the web page. It is not possible to interact with, however you can still go to another page via the Back button so the application itself as a whole is responsive.