It turns out that the Variables window was indeed being hidden.
I took your suggestion -- and tried some other things, the dialog finally reappeared. In the computer's Display settings, I switched the primary monitor from the built-in laptop monitor to the secondary and then back again.
I believe the Adobe engineers are already aware of it as an issue. This is because, after I posted my original post, I found a similar question asked in a different post. The user there was with version 6. An Adobe engineer provided a very general answer, however. Yours is better.
Thanks, Rick, for the great response. 