But it's not Firefox: I see exactly the same artifacts with Safari 13.1.2. (Safari is also much slower with this demo than Firefox.)

Surprisingly, it depends which GPU it starts with, not the one it's running on. Some Macbooks have two GPUs, only one active at a time.

If Wetlands is started with the Intel GPU active, it looks fine, and continues to look fine if the GPU is switched to nVidia. But if started with the nVidia GPU active, it looks a mess, lots of colour noise, and continues to look that way if the GPU is switched to Intel.

I've checked all combinations and found the same on both Firefox and Safari. So I guess it's probably the underlying OpenGL driver. I'm relieved, this means it's not a faulty GPU :-)