JDK project jigsaw is proposing changes to the layout of the JDK that
would initially (at least) move the JDK fonts from lib/fonts to conf/fonts.
Since FX digs into the JDK to get fonts it needs to be updated to be
aware of this potential change and so to be ready for it.
That is the minimum work needed here.
Further down the road as jigsaw may consider the on-disk image layout to
be completely private and perhaps not even accessible in a fixed stable
manner, then a modular JavaFX might need to explicitly depend on a JDK
fonts module and use an API from that to determine the location of the fonts.
Either that or FX duplicates the fonts it needs :-(