I can confirm EXACTLY the same experience on my Win10 PC!
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If my ethernet cable is connected, my port option is greyed out.
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If I disconnect my ethernet cable, I can see all my ports!
What kind of nonsense is this? This would indicate that some network service is required to be up and running if we are to do development on an online PC? Why would that be? Exactly what info is being exchanged?
Its also somewhat "comforting" that the Linux IDE is currently behaving the same as the Windows IDE...
Perhaps some online Arduino service is offline? If so, why do we have this dependency?
(Thanks for a very astute observation!)