Bug - Invisible Window (blue outline only)
[This answer not final until this sentence removed]

As I'm sure you know, troubleshooting is a process of elimination and often requires patience.

I would start with testing your MBP in Safe Mode to see how it behaves both during Safe Mode and afterwards.

Boot into Safe Mode

Follow these steps to boot your MBP into Safe Mode:


Fully shut down your MBP
Restart your MBP
Immediately press the Shift key and keep it down 
Let go of the Shift key when you see the login window (NOTE: If you have FileVault enabled you may need to log in twice).
Take a note of what happens (i.e. use your MBP long enough to establish if the problem reappears.)
Exit Safe Mode by restarting your MBP as normal
Test again to see if the problem reappears


Let me know how you went and we'll go from there.