~~The simulation clock is based on the actual frame rate achieved via requestAnimationFrame().~~ Edit: Actually, it looks like the physics portion takes consta

~~The simulation clock is based on the actual frame rate achieved via requestAnimationFrame().~~ Edit: Actually, it looks like the physics portion takes constant (but implicit) time steps each frame, but the time that a circle enters the simulation is based on the actual frame rate, which means when a given circle appears, the lines could be in a different location from previous runs.