I was having an issue with a program behaving as though there were two monitors. The ATI video card had dual ports but only one monitor. I tried many things like reinstalling drivers and making sure the display setting is not extended or duplicated.
Ultimately it was the VNC Mirror Driver. I was able to fix and then “break” again by toggling between Enabled/Disabled under Device Manager > Display Adapters > VNC Mirror Driver > right-click Disable. So I uninstalled the mirror driver.