Currently GTK3 is the default on Linux. However, there is build logic that will skip building glassgtk3 if the gtk3 development libraries are not installed. This creates a situation where the build will succeed, but the resulting JavaFX library will fail to run unless you explicitly force GTK2 with "-Djdk.gtk.version=2".

Having GTK3 be optional at build time made sense when GTK2 was the default, but now that GTK3 is the default, this logic does more harm than good. The build should fail if the GTK3 libraries are not available, as is done for other required libraries (e.g., OpenGL).