Fresh setup of Xubuntu 16.04.3 (64bit).
Graphics card is an Nvidia GF119 (GT 520 chipset).
Using the Xorg driver "nouveau" works fine, except when using two screens (dualhead, dualscreen):
The "--pos" option of xrandr offsets the mouse cursor correctly, but shifts the windows so that the cursor is relatively at the wrong position (according to the display offset configured in xrandr).
The same problem happens when using XFCE's Display settings to position the screens.
Unfortunately, I had to install nvidia's proprietary driver...
Using Ubuntu's "Additional drivers" tool, I selected the following nvidia driver:
NVIDIA legacy binary driver - version 304.135 from nvidia-304 (proprietary)
Now it works...