Arduino UNO R4 Minima - I can send keystrokes, or I can control a stepper motor. But I can't do both things in the same sketch

[Paul_KD7HB]

@CesareBrizio

Now, it looks like Serial.available() is always > 0, even when I'm not sending any character via the Serial Monitor of the Arduino IDE.

So, read and print the extra characters and see what they are. Print in HEX.