top row of pixels corrupt when right hand monitor is positioned above left monitor

When my right hand monitor is positioned above the left hand monitor, the top row of pixels on the left hand monitor appears as garbage. This does not occur when they are aligned.

May or may not be related to this old bug with the same symptoms:
https://bugs.launchpad.net/ubuntu/+source/unity-2d/+bug/846335

Occurs with standard kernel as well as my current linus mainline kernel.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: unity 7.3.2+15.04.20150410.1-0ubuntu1
Uname: Linux 4.0.0+ x86_64
.tmp.unity.support.test.0:

ApportVersion: 2.17.2-0ubuntu1
Architecture: amd64
CompizPlugins: No value set for `/apps/compiz-1/general/screen0/options/active_plugins'
CompositorRunning: compiz
CompositorUnredirectDriverBlacklist: '(nouveau|Intel).*Mesa 8.0'
CompositorUnredirectFSW: true
CurrentDesktop: Unity
Date: Tue Apr 21 16:49:59 2015
DistUpgraded: Fresh install
DistroCodename: vivid
DistroVariant: ubuntu
GraphicsCard:
 Intel Corporation Xeon E3-1200 v3/4th Gen Core Processor Integrated Graphics Controller [8086:0412] (rev 06) (prog-if 00 [VGA controller])
   Subsystem: Gigabyte Technology Co., Ltd Device [1458:d000]
 Advanced Micro Devices, Inc. [AMD/ATI] Cayman PRO [Radeon HD 6950] [1002:6719] (prog-if 00 [VGA controller])
   Subsystem: Gigabyte Technology Co., Ltd Device [1458:21fd]
MachineType: Gigabyte Technology Co., Ltd. Z97X-Gaming G1 WIFI-BK
ProcKernelCmdLine: BOOT_IMAGE=/@/boot/vmlinuz-4.0.0+ root=UUID=e6ab555f-b511-4192-9e40-222c578906bc ro rootflags=subvol=@ usbhid.quirks=0x1B1C:0x1B11:0x20000400,0x1B1C:0x1B12:0x20000400 splash quiet crashkernel=384M-:128M vt.handoff=7
SourcePackage: unity
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 03/27/2014
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: F1
dmi.board.asset.tag: To be filled by O.E.M.
dmi.board.name: Z97X-Gaming G1 WIFI-BK
dmi.board.vendor: Gigabyte Technology Co., Ltd.
dmi.board.version: x.x
dmi.chassis.asset.tag: To Be Filled By O.E.M.
dmi.chassis.type: 3
dmi.chassis.vendor: Gigabyte Technology Co., Ltd.
dmi.chassis.version: To Be Filled By O.E.M.
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvrF1:bd03/27/2014:svnGigabyteTechnologyCo.,Ltd.:pnZ97X-GamingG1WIFI-BK:pvrTobefilledbyO.E.M.:rvnGigabyteTechnologyCo.,Ltd.:rnZ97X-GamingG1WIFI-BK:rvrx.x:cvnGigabyteTechnologyCo.,Ltd.:ct3:cvrToBeFilledByO.E.M.:
dmi.product.name: Z97X-Gaming G1 WIFI-BK
dmi.product.version: To be filled by O.E.M.
dmi.sys.vendor: Gigabyte Technology Co., Ltd.
version.compiz: compiz 1:0.9.12.1+15.04.20150410.1-0ubuntu1
version.ia32-libs: ia32-libs N/A
version.libdrm2: libdrm2 2.4.60-2
version.libgl1-mesa-dri: libgl1-mesa-dri 10.5.2-0ubuntu1
version.libgl1-mesa-dri-experimental: libgl1-mesa-dri-experimental N/A
version.libgl1-mesa-glx: libgl1-mesa-glx 10.5.2-0ubuntu1
version.xserver-xorg-core: xserver-xorg-core 2:1.17.1-0ubuntu3
version.xserver-xorg-input-evdev: xserver-xorg-input-evdev 1:2.9.0-1ubuntu2
version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:7.5.0-1ubuntu2
version.xserver-xorg-video-intel: xserver-xorg-video-intel 2:2.99.917-1~exp1ubuntu2build1
version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:1.0.11-1ubuntu2build1
xserver.bootTime: Tue Apr 21 16:40:39 2015
xserver.configfile: default
xserver.errors:

xserver.logfile: /var/log/Xorg.0.log
xserver.version: 2:1.17.1-0ubuntu3
xserver.video_driver: radeon