All screens moving, jumping up on Mac

When using Safari, Finder, or Word, the page moves (jumps) up slightly. It's doing it right now as I type. I'm on an iMac desktop (mid 2010) using macOS High Sierra version 10.13.6. Using a keyboard without a trackpad and a wireless bluetooth mouse. I've replaced mouse batteries. I've cleared history and removed all cookies in Privacy Settings. I've shut down and restarted. Problem persists. I had this issue about a year ago, and resolved it, and cannot remember how I did that! Any help greatly appreciated!