I have these problems on a W520 (i.e. Nvidia driver), but on Arch Linux.
Still, what I have found, and a workaround:

The Nvidia driver will try to make the whole size of the screen (set
with xrandr --fb or by placing monitors) accessible with the monitors it
has, and if need be with panning. It will ignore or overwrite all
attempts to disable that panning if the driver thinks you need it. (It's
a feature, they brag about it under question "How are virtual screen
dimensions determined in TwinView?" at
http://us.download.nvidia.com/XFree86/Linux-
x86_64/346.35/README/configtwinview.html

On my setup, I use two external monitors that are connected to the Nvidia card 
along with the built-in monitor that is connected to the internal Intel chip.  
The intel chip servers as a Sink provider for the Nvidia driver, so all three 
monitors form a single screen, like so:
xrandr --setprovideroutputsource modesetting NVIDIA-0
xrandr --fb 5760x1200
   --output LVDS-1-0 --mode 1920x1080 --pos 3840x120
   --output VGA-0 --mode 1920x1200 --pos 1920x0
   --output DP-3 --mode 1920x1200 --pos 0x0

The nvidia driver (so it seems) will accept that the whole screen is now
5760x1200 big, but only uses the two monitors that are actually
connected to the card to "cover" that area with panning. So the right-
most laptop display worked fine, while the two monitors would pan along
left and right.

I have worked around this by abusing that while the Nvidia driver ignores or 
overwrites the core panning parameters, it doesn't meddle with the panning 
border. I modified the command above like so:
xrandr --fb 5760x1200
--output LVDS-1-0 --mode 1920x1080 --pos 3840x120 --panning 
5760x1200+0+0/0x0+0+0/-3840/0/0/0 
--output VGA-0 --mode 1920x1200 --pos 1920x0 --panning 
5760x1200+0+0/0x0+0+0/-1920/0/-1920/0 
--output DP-3 --mode 1920x1200 --pos 0x0 --panning 
5760x1200+0+0/0x0+0+0/0/0/-3840/0

This sets the borders that trigger the panning so far out that they are
all off screen. So panning is technically still enabled but impossible
to trigger.

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https://bugs.launchpad.net/bugs/1326688

Title:
  Can't disable panning on multiple monitors

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