Okay, now what?

There’s a set of sample images over on No, Dave, it’s just you that, in theory, will help you determine if your Apple laptop display has 24-bit or 18-bit color. I can detect no banding in the 24-bit image, so I must have one of the (allegedly few) good MacBooks, right?

Wrong. Before trying this test, I used SwitchResX to determine that my MacBook has an AU Optronics B133EW01, which is definitely an 18-bit display.

I suspect that all Apple (and most other) laptops have 18-bit displays, and the real news here is that Apple bought some lemons.