The thing is the window is positioned wrongly. It isn't positioned at 0, 0. It gets moved 9 pixels across on Windows 10
The window is positioned correctly. The issue is purely visual; would the border not be 100% transparent, everybody would be happy. You just
think it is offset by 9 pixels, but that's because you can't see the border. But it's definitely there.
It's literally as simple as checking whether the host OS is Windows 10 and if so then minus 9 pixels on the x. Instead of confusing people and having wrong displays on other computers.
That would be a quick and dirty "fix".
Can you be sure that Windows 10 will not change this in a future update? Can you be sure that users can't change this with an option? Can you be sure that users can't change this by applying a different theme?
I would even say that offsetting it by -9 pixels would be a bug, because the borders would become out of reach, and the user wouldn't be able to resize the window by the left side.