My screen setup is as follows:
xrandr --output DVI-I-1 --auto
xrandr --output DVI-I-2 --right-of DVI-I-1 --rotate left --auto
ie. two screens side by side, one rotated left.
With xorg from -current, mouse is jailed to only the first 1080 lines of the DVI-I-2 screen and I'm unable to access the whole of the second screen. Downgrading to x/ packages from 14.0 fixed the issue.