Neat. The phenomenon is called Rayleigh fading, and is well known, but I've never seen it mapped as clearly. Rayleigh fading is often viewed statistically, and some article even call it "random". As shown here, it's not random at all; it's deterministic and repeatable. It's determined by the position of transmitter, receiver, and reflective and absorbent objects in the environment.

This is why most WiFi devices have two antennas and a diversity receiver.

Spread-spectrum systems are resistant to Rayleigh fading, because multipath nulls are frequency-specific; the two paths have to be half a wavelength different in length to get cancellation. So it seems surprising at first to see this with WiFi, which is a spread-spectrum system.

There's a reason for that. Cancellation becomes less of a problem as you get further from a spread spectrum transmitter, because the phase difference is related to the product of the frequency change times the number of wavelengths along the path. (This really needs pictures.) Once you're far enough away that the spreading moves the nulls at least a half wavelength, the effect is that a fixed percentage of data is lost, and error correction deals with that. In this demo, transmitter and receiver are very close, so the nulls are strong.