should be a popup menu, that is a menu that is displayed using display-popup-menu.
popup-menu-force-popdown pops it down, in the same way that pressing
Cancel would normally do.
popup-menu-force-popdown can be called from any process. In particular, it can be called from a timer without worrying on which process it is actually executed. For examples of using timers in CAPI, see Examples using timers to implement "animation".
CAPI User Guide and Reference Manual (Macintosh version) - 3 Aug 2017