Chosen Solution
Same issue with 2015 13”… when using Windows 10 it works without issues, so the theory of TCON still stands? What is windows doing differently? It’s in full resolution using latest bootcamp drivers. Of course macOS works fine too as long as there is a monitor on the HDMI port
Then you have a great MS Windows system! Sniff ;-{ The issue is the scan rate of the screen. Think of it this way your car has three gears and over-drive (4th) the gas you put in its how far you can run the gears. The Windows drivers only go to third, whereas the macOS native drivers can push into fourth! But that gear is messed up! What we are really talking about is the scan rate of the display resolutions: 2560-by-1600 (Retina); Scaled resolutions: 1680 by 1050, 1440 by 900, & 1024 by 640 pixels. The Windows drivers will get you to 1680 by 1050, to get 2560-by-1600 you need to use macOS natively and this uses a higher scan rate (kHz) the circuitry is damaged within the displays T-CON logic.
