Popups in the middle of the dual screens

[dargaud]

Hello all,
an annoying issue. I've googled for it and found plenty of people with the same problem but no solution. I have dual monitors and when there's a popup window, it shows up in the middle, half on one monitor, half on the other.

Is there a way to get it to show in the middle of a specific screen ? I have kde and the nVidia non-free driver, if that matters.
Thanks.

[komputes]

I think the nvidia binary driver has an option to star an X server on each display, mwaning you can have two webpages open and they will most likely have popups placed in the middle of each screen.