Re: Screen bug with nvidia driver
because if you don't use modes the monitor and graphic driver are free to find the optimal solution via ddc/edid.
As soon as you start setting stuff you override this.
That is why ubuntu's xorg.conf is an example of a crappy xorg.conf. Full of unneeded, harmfull modelines.
btw, if you change resolution on the fly with xrandr or krandrtray you won't get black lines or the 'magnfying glass' effect, you'll get with modes set in xorg.conf.