[ubuntu] wondering mouse cursor on laptop with external 24" external monitor

Hi Community:

I upgraded my wife's laptop from 14.04 to 16.04 today.

She has an exteal 24" monitor, and I set it up with the "Displays" on "System Settings."

I was successful in getting 16.04 to detect the 24" monitor.

However, the mouse cursor wonders from the 24" monitor to the laptop monitor.

How do I restrict the cursor to stay on the 24" monitor?

Thanks,
Old Jimma from the Old Country