Thanks Robert.
We've already been through quite a lot of testing with ESRI Tech Support and tested all of those things that you mention: different PC's, different hardware, different OS's, even using only ESRI services for data, etc. It's not them any of them specifically and ESRI is stumped, so far.
It is reproducible, though uncommon. I suspect that's because it happens only when ArcGIS Pro is minimized, so that users just see a mysterious black box and don't realize that it's ArcGIS Pro that is causing it. It also only happens when a map or tool frame is separated from the main window, which not everyone does.
So, I suspect that there are other users, perhaps many, who see this problem but don't recognize the cause, so they just restart their PC without ever solving it.
If you are not experiencing the problem, that's GREAT! 🙂 However, please do not harass people who are experiencing the problem and who are just trying to raise awareness of it and identify other users who have also seen it.
I asked for kudos specifically and only from people who are also experiencing this problem in order to help get it onto ESRI's radar. That's what kudos are for.
I hope this answers your questions.
Also, Robert. If you're not experiencing this problem, please be so kind as to add you're hardware and OS configuration. That may help ESRI narrow the range of suspects and find the cause of this. Thanks!