Arduino GUI consumes CPU when serial monitor is opened while board is unattached

Dear all,

I have noticed the following misbehaviour of Arduino GUI v1.8.9. I am monitoring the messages produced by my board Arduino Nano (ATmega328P) using “Serial Monitor”.

Once the board is disconnected (USB is unattached) Arduino GUI starts to consume CPU. Not much, but greedy enough to force CPU fan to rotate faster. If I close “Serial Monitor” window, Arduino GUI is OK again.

Does somebody observe the same problem?

This is a known bug:

It has already been fixed, but there has not been a production release of the Arduino IDE since that time. If you want to test the fix, you can try using the hourly build of the Arduino IDE: