iTunes won't display window or respond to keyboard

iTunes 12.7.3; MacBook Air circa 2011, running 10.13.3

I recently turned on both Apple Music and then iCloud Music Library. After turning on Apple Music, iTunes worked fine on my machine. Not sure whether it's coincidental or causal, but after I turned on iCloud Music Library, iTunes is essentially non-responsive to typical keyboard actions: doesn't respond to command-tab (that is, the app doesn't assert focus or display its window when I command-tab to it; even if I hide all other apps, the only way I can get iTunes to display a window is by clicking on a menu item that forces a dialog (such as "create new playlist"). When I do get iTunes menu bar visible, it's greyed-out until/unless I click on it (or the iTunes window, assuming I've gotten the app to pop that window).

Once I "nudge" iTunes to be visible, I can play music using the main iTunes window just fine.

I have re-installed iTunes. I have re-booted the Mac.

What the heck is going on here?