I think the main difference is that with multi-monitor support, some of the monitors can have negative coords.

I was wondering how the OS reported the second monitor position to the application. I've never coded for multiple monitors, so I didn't know.

Maybe there's some logic in the built-in functions that try to make sure a window isn't openning off-screen (e.g. no negative coords) that should be re-writen for multi-monitor support.

No, it's just that I deliberately reset the saved window position if it comes up as being off the screen (i.e, negative number or greater than the screen width or height). This is for people who change their desktop resolution. If they exit the program at a high resolution while the window is off to the side, then start it again at a lower resolution, then the window will be off screen and they would have no way to get it back on the screen.

Perhaps I just need to make that part of it optional. Add a checkbox to the config screen that disables this window-resetting code. Then maybe it would just work for those with multiple monitors.