So I got a surprise today.

Grass is not killed off by being under quicksand.

This means, that the light is passing through quicksand, just like stairs or slabs, or chests, to keep the grass alive.

Which means, just like tilled soil, cracked sand underneath quicksand is exposed to sunlight, and spreads ...

I don't know if that's intentional or not.

This is no longer the case in the 3.0 release.