MacBook – How to determine which app is running on the Touch Bar in MBP

The new shortcut is Command-Dim Brightness. It even makes sense in the following way:

Assuming that you take advantage of the touch bar, and it displays the system controls instead of the function keys then the hand gesture is exactly the same as before when you would have pressed Command-F1 since the Dim Brightness button is where F1 would be:

The annoying thing is that if you have an application-aware touch bar configuration, then you have to bring up the system controls touch bar configuration first.

This is why, in some cases, the Display Menu (in the App Store) which adds a status item in your menu bar (and includes a Mirror Screen choice) is a good option.

I'd imagine the Touch Bar can become buggy for the same reasons that any computer becomes buggy, so what you could do is use Touch Bar Simulator to see what your Touch Bar thinks it's displaying, thereby narrowing it down to if the issue is software or hardware. If it's software, try the usual stuff like disk repair and reinstalling macOS. If it's hardware, it may be time to visit an Apple Store...