It's not apparent what is happening...
Consider the animation below. (Tap canvas to start/pause/continue). Stare at the crosshair and observe: is there more than one color of dot in this animation?
Only our P5JS programmer knows for sure!
Play with the settings and you be the judge of what's going on!
Meanwhile, read up on Lilac Chaser and its history at Wikipedia.
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Click canvas to start/pause/continue