Place a window in the centre of the screen, and another window somewhere else, and the mouse pointer in the centre, and try to use alt-tab to switch focus. The pop-up disappears when you let go of alt-tab and I think that makes sawfish give focus to the window that the mouse pointer is 'now' over.
Sent upstream to: http://bugzilla.eazel.com/show_bug.cgi?id=8384 Closing on Red Hat level.