Indeed, an open cell does cause the same symptom, but I have the
problem when I am not in edit mode.

Two additional symptoms:

1. Excel remains functional.
2. After visiting the unresponsive VBE, if I go back to Excel, the bar
along the top of the Excel window is dimmed as if the Excel were not my
active window. It remains dimmed even as I type into cells in Excel.

I must be doing something stupid, but I can't figure out what it is!