Can javapackager (https://docs.oracle.com/javase/9/tools/javapackager.htm#JSWOR719) package Windows app from a Mac, or does one need to actually install Windows?
From Oracle docs:
Self-contained application packages are platform-specific and can only be produced for the same system on which you build. To deliver self-contained application packages on Windows, Linux, and OS X, you must build your project on all three platforms.
https://docs.oracle.com/javase/8/docs/technotes/guides/deploy/self-contained-packaging.html#A1307236