Look like it doesn't work on Safari (or at least Safari Version 13.0 (15608.1.37) on Catalina). The buttons are below the viewport and I can't scroll to them. It worked fine in Chrome.
Confirmed on Safari 12.1.1--albeit different than you mention. Things work fine if I'm fullscreen on a 15", but if I reduce the window size, I see the color palette & buttons cover the image with no option to minimize or move them.