12.04 nvidia Xinerama dual screen mouse jumps when crossing left across display edge

I recently updated my system to 12.04; it has two screens driven by an NVidia GeForce 8400GS graphics card. Xinerama is enabled. When I move the mouse left across screens, it jumps from left edge of one to left edge of the other. It does not do this in the reverse direction. This right-to-left behavior is really weird; left-to-right is okay.

More details:
* xorg.conf shows screen 0 is left, screen 1 is right, and Xinerama is enabled. (See below.)
* Moving the mouse on the desktop from screen 1 to 0 (i.e., to the left), it leaves the left edge of screen 1 and shows up on the left edge of screen 0; it does not glide across screen 0 to get there... unless I move across the edge at !very! slow speed; I can then pick up speed after crossing, and it behaves normally. (Yes, speed sensitive.)
* It does not do this when moving right (screen 0 to screen 1). In that case, it mouse glides across the desktop with no problem.

To add some confusion, running 'nvidia-settings', it talks about a single screen and two displays.
* X Screen 0 shows dimensions 2960x1050, and displays Dell 2005FPW (DFP-0) and Sharp LL-172C-B (CRT-1).
* GPU 0 shows:
  - CRT-1 (Sharp) refresh rate (60.02 Hz), but no dimensions;
  - DFP-0 (Dell) resolution 1680x1050 and refresh rate of 59.88 Hz, 'Full Force GPU Scaling', GPU Scaling Method: 'Stretched'. (But it looks fine to me.)

Below are the xorg.conf file and a fragment of the Xorg.0.log file.

-------- xorg.conf below ----
# nvidia-settings: X configuration file generated by nvidia-settings
# nvidia-settings: version 295.33 (buildd@allspice) Fri Mar 30 13:37:33 UTC 2012

# nvidia-xconfig: X configuration file generated by nvidia-xconfig
# nvidia-xconfig: version 295.40 (<email address hidden>) Thu Apr 5 22:40:54 PDT 2012

Section "ServerLayout"

# Removed Option "Xinerama" "0"
    Identifier "Layout0"
    Screen 0 "Screen0" 0 0
    Screen 1 "Screen1" RightOf "Screen0"
    InputDevice "Keyboard0" "CoreKeyboard"
    InputDevice "Mouse0" "CorePointer"
    Option "Xinerama" "1"
EndSection

Section "Files"
EndSection

Section "InputDevice"

    # generated from default
    Identifier "Mouse0"
    Driver "mouse"
    Option "Protocol" "auto"
    Option "Device" "/dev/psaux"
    Option "Emulate3Buttons" "no"
    Option "ZAxisMapping" "4 5"
EndSection

Section "InputDevice"

    # generated from default
    Identifier "Keyboard0"
    Driver "kbd"
EndSection

Section "Monitor"
    Identifier "Monitor0"
    VendorName "Unknown"
    ModelName "DELL 2005FPW"
    HorizSync 30.0 - 83.0
    VertRefresh 56.0 - 75.0
    Option "DPMS"
EndSection

Section "Monitor"
    Identifier "Monitor1"
    VendorName "Unknown"
    ModelName "Sharp LL-172C-B"
    HorizSync 24.0 - 82.0
    VertRefresh 55.0 - 77.0
EndSection

Section "Device"
    Identifier "Device0"
    Driver "nvidia"
    VendorName "NVIDIA Corporation"
    BoardName "GeForce 8400GS"
    BusID "PCI:1:0:0"
    Screen 0
EndSection

Section "Device"
    Identifier "Device1"
    Driver "nvidia"
    VendorName "NVIDIA Corporation"
    BoardName "GeForce 8400GS"
    BusID "PCI:1:0:0"
    Screen 1
EndSection

Section "Screen"
    Identifier "Screen0"
    Device "Device0"
    Monitor "Monitor0"
    DefaultDepth 24
    Option "TwinView" "0"
    Option "TwinViewXineramaInfoOrder" "DFP-0"
    Option "metamodes" "DFP: nvidia-auto-select +0+0"
    SubSection "Display"
        Depth 24
    EndSubSection
EndSection

Section "Screen"
    Identifier "Screen1"
    Device "Device1"
    Monitor "Monitor1"
    DefaultDepth 24
    Option "TwinView" "0"
    Option "TwinViewXineramaInfoOrder" "CRT-1"
    Option "metamodes" "CRT: nvidia-auto-select +0+0"
    SubSection "Display"
        Depth 24
    EndSubSection
EndSection

Section "Extensions"
    Option "Composite" "Disable"
EndSection
-------- xorg.conf above --------

-------- Xorg.0.log fragment below --------
 [ 18.296] (==) Log file: "/var/log/Xorg.0.log", Time: Fri Aug 10 07:06:57 2012
 [ 18.322] (==) Using config file: "/etc/X11/xorg.conf"
 [ 18.322] (==) Using system config directory "/usr/share/X11/xorg.conf.d"
 [ 18.422] (==) ServerLayout "Layout0"
 [ 18.422] (**) |-->Screen "Screen0" (0)
 [ 18.422] (**) | |-->Monitor "Monitor0"
 [ 18.422] (**) | |-->Device "Device0"
 [ 18.422] (**) |-->Screen "Screen1" (1)
 [ 18.422] (**) | |-->Monitor "Monitor1"
 [ 18.422] (**) | |-->Device "Device1"
 [ 18.599] (**) |-->Input Device "Keyboard0"
 [ 18.599] (**) |-->Input Device "Mouse0"
 [ 18.599] (**) Option "Xinerama" "1"
 [ 18.599] (==) Automatically adding devices
 [ 18.599] (==) Automatically enabling devices
 [ 18.599] (**) Xinerama: enabled
(...snipped...)
 [ 18.830] (II) LoadModule: "dbe"
 [ 18.830] (II) Loading /usr/lib/xorg/modules/extensions/libdbe.so
 [ 18.858] (II) Module dbe: vendor="X.Org Foundation"
 [ 18.858] »··compiled for 1.11.3, module version = 1.0.0
 [ 18.858] »··Module class: X.Org Server Extension
 [ 18.858] »··ABI class: X.Org Server Extension, version 6.0
 [ 18.858] (II) Loading extension DOUBLE-BUFFER
 [ 18.858] (II) LoadModule: "glx"
 [ 18.858] (II) Loading /usr/lib/x86_64-linux-gnu/xorg/extra-modules/libglx.so
 [ 20.109] (II) Module glx: vendor="NVIDIA Corporation"
 [ 20.109] »··compiled for 4.0.2, module version = 1.0.0
 [ 20.109] »··Module class: X.Org Server Extension
 [ 20.109] (II) NVIDIA GLX Module 295.40 Thu Apr 5 21:57:38 PDT 2012
 [ 20.109] (II) Loading extension GLX
 [ 20.109] (II) LoadModule: "record"
 [ 20.109] (II) Loading /usr/lib/xorg/modules/extensions/librecord.so
 [ 20.152] (II) Module record: vendor="X.Org Foundation"
 [ 20.725] »··compiled for 1.11.3, module version = 1.13.0
 [ 20.726] »··Module class: X.Org Server Extension
 [ 20.726] »··ABI class: X.Org Server Extension, version 6.0
 [ 20.726] (II) Loading extension RECORD
 [ 20.726] (II) LoadModule: "dri"
 [ 20.726] (II) Loading /usr/lib/xorg/modules/extensions/libdri.so
 [ 21.205] (II) Module dri: vendor="X.Org Foundation"
 [ 21.205] »··compiled for 1.11.3, module version = 1.0.0
 [ 21.205] »··ABI class: X.Org Server Extension, version 6.0
 [ 21.205] (II) Loading extension XFree86-DRI
 [ 21.205] (II) LoadModule: "dri2"
 [ 21.205] (II) Loading /usr/lib/xorg/modules/extensions/libdri2.so
 [ 21.613] (II) Module dri2: vendor="X.Org Foundation"
 [ 21.613] »··compiled for 1.11.3, module version = 1.2.0
 [ 21.613] »··ABI class: X.Org Server Extension, version 6.0
 [ 21.613] (II) Loading extension DRI2
 [ 21.613] (II) LoadModule: "nvidia"
 [ 21.613] (II) Loading /usr/lib/x86_64-linux-gnu/xorg/extra-modules/nvidia_drv.so
 [ 21.933] (II) Module nvidia: vendor="NVIDIA Corporation"
 [ 21.933] »··compiled for 4.0.2, module version = 1.0.0
 [ 21.933] »··Module class: X.Org Video Driver
 [ 21.986] (II) NVIDIA dlloader X Driver 295.40 Thu Apr 5 21:38:35 PDT 2012
 [ 21.986] (II) NVIDIA Unified Driver for all Supported NVIDIA GPUs
 [ 22.004] (++) using VT number 7

 [ 22.006] (II) Loading sub module "fb"
 [ 22.006] (II) LoadModule: "fb"
 [ 22.037] (II) Loading /usr/lib/xorg/modules/libfb.so
 [ 22.098] (II) Module fb: vendor="X.Org Foundation"
 [ 22.098] »··compiled for 1.11.3, module version = 1.0.0
 [ 22.098] »··ABI class: X.Org ANSI C Emulation, version 0.4
 [ 22.098] (II) Loading sub module "wfb"
 [ 22.098] (II) LoadModule: "wfb"
 [ 22.098] (II) Loading /usr/lib/xorg/modules/libwfb.so
 [ 22.124] (II) Module wfb: vendor="X.Org Foundation"
 [ 22.124] »··compiled for 1.11.3, module version = 1.0.0
 [ 22.124] »··ABI class: X.Org ANSI C Emulation, version 0.4
 [ 22.124] (II) Loading sub module "ramdac"
 [ 22.124] (II) LoadModule: "ramdac"
 [ 22.124] (II) Module "ramdac" already built-in
 [ 22.126] (WW) NVIDIA: Xinerama is enabled, so RandR has likely been disabled by the
 [ 22.126] (WW) NVIDIA: X server.
-------- Xorg.0.log fragment above --------