Large Rectangle follows Cursor across screen
After upgrade to High Sierra 10.13.1, I seem to have a large 25cm x 14cm rectangle now following the pointer across the screen. It always stays on the screen even when the pointer moves to the corners. It seems to be a kind of tracking tool which may have been left 'ON' during development and has been rolled out in the production version released. Does anyone know if a switch/function that can turn it off? Perhaps a patch fix is needed?
Mac mini (Late 2012), macOS High Sierra (10.13.1), wireless keyboard + trackpad