A predicate for whether any CAPI window is currently displayed.
is a predicate for whether any CAPI window (other than dialogs) is currently displayed.
LispWorks CAPI User Guide
for a description of the CAPI toolkit which allows you to write graphical user interfaces in Lisp.
Tools in the LispWorks IDE are all CAPI windows.
LispWorks User Guide and Reference Manual - 21 Dec 2011