Last edited: 28 Feb 2018
The UI controls are functional while an image is drawn; you don't have to wait until it's done. Clicking on the "Approximation: On/Off" label toggles approximation mode, which shows an increasingly finegrained approximation of the final image. This is useful to get a first idea of what an image looks like, and zoom/pan without having to wait for the whole image to be finished.
The source code can be found here. This is some of the earliest Java code I ever wrote, so the code quality is not really what I would like it to be nowadays :-)