A slot in a CAPI interface definition can now have an initform that instantiates a pane, for example
:initform (make-instance 'capi:text-input-range
:title "Title" :callback 'my-callback)
In LispWorks 6.0 and earlier an error would be signalled when displaying such an interface.
LispWorks Release Notes and Installation Guide - 23 Dec 2011