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There is a bug in the ESP8266 Arduino core that averwrites the overlap
mode setup and hold bits for the hardware driven SPI SS line.  The
TFT_eSPI library has been patched as a work-around.

In overlap mode to maintain the required setup and hold times at 80MHz
ESP8266 CPU clock the SPI rate must be 27MHz or less. At 160MHz CPU
clock the SPI rate can be 40MHz.