On macOS High Sierra 10.13.3, clicking the "View Plugin" button for Zyn-Fusion ( http://zynaddsubfx.sourceforge.net
results in a black screen. Plugin receives MIDI and outputs audio just fine. Plugin GUI works fine in Reaper, MuLab and Ardour. I reported it to the Zyn-Fusion developer, Mark McCurry and he said, "I'm guessing that Audio Plugin Player puts the openGL context into an unexpected state as the GUI seems to work with other hosts."