The OpenVPMS Editor Launcher started by External Edit uses SWT to locate OpenOffice on the local machine.
This doesn't work on OSX, and apparently relates to the requirements of SWT on OSX is that the JVM be launched with -XstartOnFirstThread.
This is supplied in the externaledit.jnlp file i.e.:
Despite these reports:
it is possible to get -XstartOnFirstThread passed to the JVM if there is no <java...> element in the main resource.
However the JVM terminates before executing the main line, with no errors.
A corresponding bug report appears to be: https://bugs.openjdk.java.net/browse/JDK-8147056