Yes, the "Source" window could have been undocked and hiding. I had considered that possibility and spent a lot of time trying to find the errant window. I never considered a single monitor test. I assumed that closing and reopening PS would reset the docking status, but that never worked.
I have two indentical laptops with Intel and Nvidia graphics (one of which is dedicated to PS). Attached to the PS laptop are two external monitors. When I did a print screen I ended up with a picture that contained all three monitors. Using crop I removed two of the monitors (one to the left of my PS monitor) before posting the screenshot. That may account for the black strip.
I did do the Reset as suggested and now, again, have the "Source" window and "two-box" icon. In retrospect, first I should have tried with a single monitor environment.
Thank you.