Mac command-tab keeps cycling in endless loop

Mac command-tab keeps cycling in endless loop instead of moving one application icon at a time with each tap of tab key. It's as if I held the tab key down.

I have a 2014 iMac with macOS Big Sur but I don't that's a factor. Command-Tab worked great until today.

I've had this before and usually was that I had a weight on the touchpad holding it down, so my fault. This time I have disabled every mouse and every keyboard connected to the Mac one at a time and the behavior persists. I Rebooted the Mac twice to no avail.

What else should I look for?

Thanks much. -Deac