Comment 9 for bug 1326688

I found a different work around that works consistently for me. I'm actually using gentoo linux with xorg-server-1.17.4, gnome-3.18.0, and xrandr-1.4.3. Looks to me like Thomas C. Blank was on the right track.

I have an NVIDIA card with a 1680x1050 monitor and a 1920x1200 monitor. And an Intel card with a 1920x1080 monitor. They are in that order left-to-right.

First, I had to disable xinerama in my xorg.conf because it conflicts with xrandr.

When I first log in, the nvidia monitors are in the correct orientation, but I have to connect the Intel output source with: xrandr --setprovideroutputsource Intel NVIDIA-0

Then I can enable the output either through gnome's display settings or with xrandr(DVI-I-2 being the 1920x1200 monitor): xrandr --output HDMI2 --auto --right-of DVI-I-2

Finally, I convince panning to do what I want with another xrandr call: xrandr --output DVI-I-2 --panning 3840x1200+1680+0/1920x1200+1680+0/0/0/0/0

This panning instruction says the center monitor pans over a region that contains itself and the right monitor. But it only tracks the mouse pointer over itself. And it has no border.
"The first four parameters specify the total panning area, the next four the pointer tracking area (which defaults to the same area). The last four parameters specify the border and default to 0." -- man xrandr

That same man page continues on to say that width or height set to zero will disable panning on that axis - which doesn't work.