No, no dual monitors. Only the single built in 1920 x 1080 Asus G73S Laptop screen.
Sorry, my "half off" may have misled you, so to be accurate, the right hand side of the window opens up (every time) about 1" off the right of the screen, and the bottom 1/2" below the screen. The Left and Top are 12" and 7" onto the screen, so it's no problem to drag it back centrally, just a bit puzzling.
I use an excellent window arranging app capable of multiple screens (Gridmove... think Window Snap on steriods) and thought that might be affecting it, but no go.
I then uninstalled U.S., rebooted and reinstalled, but still the same.
The machine is very new, so I think all the graphics drivers are up to date & OK, but I suspect I may find the solution to this very minor thing as I get more familiar with it.