With focus stealing prevention set to High or Extreme, and focus policy set to Focus Follows Mouse - Mouse Precedence, clcking on the Application Menu does not normally launch the application menu.  I have not tested other policies to which focus stealing prevention applies (there's no problem using Focus Under Mouse, but focus stealing prevention does not apply to that policy).

If FSP is set to High, and focus is removed from any existing window (e. g. by clicking on empty space on the desktop), it is possible to launch the application menu.  It is not possible to do so if FSP is set to Extreme (which as aside I would really like to do, because even set to High, LibreOffice manages to steal focus every now and again in the process of opening or saving a very large and complex document; I find Medium or below to be completely unacceptable).

On occasion, the application menu appears to launch and inmediately disappear, suggesting that it expects to receive focus immediately upon launch and dismisses itself when it finds itself without having focus yet.

Other panel applets, such as the Device Notifier, launch successfully, leading me to believe that this is not an issue with KWin's handling of focus stealing prevention (ref. bug 377914).