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.
I had a similar bug happen to me. A fix I found was to make the window fullscreen and then not fullscreen. Being that you likely can't see the fullscreen button, another option if you already know the window is maximized (not fullscreen), it to put open another window of the same size, layer the invisible window on top of it, and then move the mouse over to the green fullscreen button of the new window. Because they are the same size, pressing should trigger the fullscreen of the invisible window. Then it's just a matter of un-fullscreening it.
If you don't know the window size, you can always open mission control and drag the invisible window to somewhere there isn't an existing Space, and that will also fullscreen it.