it's breaking, because ZynFusion is much more convenient than legacy ZynAddSubFX UI.
As of December 31st 2017 the full source for the Zyn-Fusion interface has been freely available on github with a combination of MIT and GPL based licenses. Additionally the Zyn-Fusion VST/LV2 versions have been ported to OSX (10.6+).
For those interested in the OSX version you can find a free demo at http://zynaddsubfx.sourceforge.net/demo.html and the full version over at https://gumroad.com/l/zyn-fusion .
Source for the GUI can be found at https://github.com/mruby-zest and build scripts can be found at https://github.com/zynaddsubfx/zyn-fusion-build and https://github.com/zynaddsubfx/zyn-build-osx .