DIAGNOSIS Screen calibration has drifted, or a static charge has built up on the capacitive screen, often from a dry environment or certain types of screen protectors.
FIX Re-calibrate the touch screen. From the main screen, hold down the "Menu" and "Fan" buttons simultaneously for about 10 seconds until the screen flashes and the thermostat reboots. This forces a touch recalibration. Use a dry, microfiber cloth to clean the screen thoroughly before doing this.
IF THAT DIDN'T WORK Perform a full factory reset. Go to Menu > Reset > Factory Reset. This wipes all your settings and schedules, so note them down first. After reboot, set up the thermostat again. If the problem persists after a reset, it's likely a hardware fault.
HEADS UP Don't use liquid cleaners or glass cleaner on the screen—they can leave a film that interferes with capacitance. Also, cheap aftermarket screen protectors can cause this; remove it to test.