Application menus go off screen the first time they are displayed

[Gary Houston]

Using the bookmarks menu in Firefox, when going to a nested bookmark (3 deep) the menu always opens partly off the right edge of my screen, but only on the first time the menu is displayed. If the menu is closed and then reopened, it appears to the left of the previous menu so will be visible.

Details: Menus are displayed in the application window title bar. Firefox is not full screen, and displayed on the right half of my monitor. Each level of bookmark menus opens to the right of the previous, and the third level is only just visible with most of the text off the right edge of the screen.

What should happen: when the menu is opened the 2nd time, it's fine, it's on the screen to the left of the previous menu. The same thing should happen the first time its opened.

Ubuntu 16.04 LTS

apt-cache policy unity:
Version table:
 *** 7.4.0+16.04.20160526.1-0ubuntu1 500

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: unity 7.4.0+16.04.20160526.1-0ubuntu1
ProcVersionSignature: Ubuntu 4.4.0-28.47-generic 4.4.13
Uname: Linux 4.4.0-28-generic x86_64
.tmp.unity_support_test.0:

ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
CompizPlugins: [core,composite,opengl,compiztoolbox,decor,vpswitch,snap,mousepoll,resize,place,move,wall,grid,regex,imgpng,session,gnomecompat,animation,fade,workarounds,scale,expo,ezoom]
CompositorRunning: compiz
CompositorUnredirectDriverBlacklist: '(nouveau|Intel).*Mesa 8.0'
CompositorUnredirectFSW: true
CurrentDesktop: Unity
Date: Thu Jul 14 11:34:53 2016
DistUpgraded: Fresh install
DistroCodename: xenial
DistroVariant: ubuntu
DkmsStatus:
 virtualbox, 5.0.18, 4.4.0-21-generic, x86_64: installed
 virtualbox, 5.0.18, 4.4.0-22-generic, x86_64: installed
 virtualbox, 5.0.18, 4.4.0-24-generic, x86_64: installed
 virtualbox, 5.0.18, 4.4.0-28-generic, x86_64: installed
GraphicsCard:
 Advanced Micro Devices, Inc. [AMD/ATI] RS780L [Radeon 3000] [1002:9616] (prog-if 00 [VGA controller])
   Subsystem: ASUSTeK Computer Inc. RS780L [Radeon 3000] [1043:8388]
InstallationDate: Installed on 2016-05-04 (70 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
MachineType: System manufacturer System Product Name
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.4.0-28-generic root=UUID=7cc92315-bf24-42a0-86c3-0d70f09b3f45 ro quiet splash vt.handoff=7
SourcePackage: unity
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 01/17/2011
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: 0801
dmi.board.asset.tag: To Be Filled By O.E.M.
dmi.board.name: M4A78LT-M-LE
dmi.board.vendor: ASUSTeK Computer INC.
dmi.board.version: Rev X.0x
dmi.chassis.asset.tag: Asset-1234567890
dmi.chassis.type: 3
dmi.chassis.vendor: Chassis Manufacture
dmi.chassis.version: Chassis Version
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvr0801:bd01/17/2011:svnSystemmanufacturer:pnSystemProductName:pvrSystemVersion:rvnASUSTeKComputerINC.:rnM4A78LT-M-LE:rvrRevX.0x:cvnChassisManufacture:ct3:cvrChassisVersion:
dmi.product.name: System Product Name
dmi.product.version: System Version
dmi.sys.vendor: System manufacturer
upstart.unity7.log:
 WARN 2016-07-14 11:33:03 unityct(io) <unknown>:0 Desktop file '/usr/share/applications/unity-activity-log-manager-panel.desktop' is using a deprecated format for its actions that will be dropped soon.
 WARN 2016-07-14 11:33:03 unityct(io) <unknown>:0 Unable to find group 'X-Activity-Log-Manager Shortcut Group'
version.compiz: compiz 1:0.9.12.2+16.04.20160526-0ubuntu1
version.ia32-libs: ia32-libs N/A
version.libdrm2: libdrm2 2.4.67-1ubuntu0.16.04.1
version.libgl1-mesa-dri: libgl1-mesa-dri 11.2.0-1ubuntu2
version.libgl1-mesa-dri-experimental: libgl1-mesa-dri-experimental N/A
version.libgl1-mesa-glx: libgl1-mesa-glx 11.2.0-1ubuntu2
version.xserver-xorg-core: xserver-xorg-core 2:1.18.3-1ubuntu2.2
version.xserver-xorg-input-evdev: xserver-xorg-input-evdev 1:2.10.1-1ubuntu2
version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:7.7.0-1
version.xserver-xorg-video-intel: xserver-xorg-video-intel 2:2.99.917+git20160325-1ubuntu1
version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:1.0.12-1build2
xserver.bootTime: Thu Jul 14 09:55:01 2016
xserver.configfile: default
xserver.devices:
 input Power Button KEYBOARD, id 6
 input Power Button KEYBOARD, id 7
 input USB Optical Mouse MOUSE, id 8
 input USB USB Keykoard KEYBOARD, id 9
 input USB USB Keykoard KEYBOARD, id 10
xserver.errors:

xserver.logfile: /var/log/Xorg.0.log
xserver.version: 2:1.18.3-1ubuntu2.2
xserver.video_driver: radeon