Stop mouse movement from moving windows off the desktop
When I move the mouse cursor sideways across the desktop, the open windows disappear to the sides of the screen. Then you have to click on the screen edge to bring them back. This has become a big annoyance and I'd like to disable this "feature". I didn't find anything in Apple Support or mouse instructions, or System Preferences. It's an iMac, 27", late 2009, macOS 10.13.6 with a wireless mouse.
Thanks for any suggestions.