After upgrading to 2.4, Page resources which have been configured as popup are now displayed with header and sidebar when viewed using Firefox (16.0.2)and Chrome (23.0.x). However, when using Internet Explorer (9.0.x) the popup page is correctly displayed without the header and sidebar.
I've tested this on a clean install of version 2.4 with default settings and no modifications. The behavior is the same. Popup pages are displayed correctly on IE but not on Firefox and Chrome.