My Macbook Pro (13 inch, 2016, 4 thunderbolt ports, touchbar) has developed strange screen fault. The entire screen turns black, for a fraction of a second, and then returns to normal, then black, then normal. It's an irregularly irregular flicker - it happens rapidly, or slowly, but not in a predictable cycle. It looks like someone is turning the power on and off again at random. The screen is basically unusable.
Things that I don't think are causing the issue:
- GPU switching. Some Macbook models have a discrete GPU and an integrated GPU, and there can be a flicker when switching between them. System report says that I have only an Intel Iris GPU.
- This does not seem to be the backlight fault that is described in some 2016 models. If I turn the backlight the whole way down, I can still see the (unilluminated) pixels flickering away in the same manner.
Workarounds that fix the problem:
- The screen is normal while the machine is booting (the Apple logo and progress bar appear as usual, no flicker).
- The screen is normal when in safe boot mode (another Ask Different user has described what seems to be a similar problem, perhaps indicating a faulty driver, or a faulty GPU function that is disabled when the driver is removed. So seemingly that problem could be fixed by identifying and disabling the appropriate Kernel extension. But, but, heres's the bit that confuses me...
- When I attach an external monitor (via Apple's Thunderbolt to HDMI adapter), BOTH screens function entirely normally.
This last point is what I consider odd. If the GPU were at fault, why would it be able to behave normally with an external monitor attached?
What should be my next troubleshooting step? I have asked Apple who advise a logic board replacement. The machine is now out of warranty and I can't afford that.