With the recent new VisibleMemberTable, we have improved detection of (JavaFX) properties based on a more detailed analysis of the relevant signatures than before. (See the comments on VisibleMemberTable.computeVisibleProperties; http://hg.openjdk.java.net/jdk/jdk/file/b5aac518b097/src/jdk.javadoc/share/classes/jdk/javadoc/internal/doclets/toolkit/util/VisibleMemberTable.java#l761)

With this is mind, it is reasonable to default the -javafx option to "on"/"true" if JavaFX is available on the (overall) module path. This could either be a check for JavaFX modules, e.g. javafx.base, for >= JDK 9, or for specific JavaFX types, e.g. javafx.beans.value.ObservableValue for any recent JavaFX/JDK.