Original Poster replied to my tweet where I asked him if he has an Apple Watch: >Hey Tonny. No I don't have an Apple Watch so it's not related. I did connect an external screen before opening the MBP though, so maybe it's related to that? Note that I can't reproduce it, happened only once so it must be a shady bug.

See https://twitter.com/BenoitLetondor/status/939164367962148864