The space of the frame window that is not taken by the original window is used for the decorative frame around the window (the border and the title bar).
The browser, when opened in this way, contains none of the regular interface except for the title bar, so as not to interrupt anything the user is trying to do.
The decorative frame and title bar (possibly including buttons) that is usually seen around windows are created by the window manager, not by the client that creates the window.