This is related to kwin - kde-window-manager - or so I hope.

Description of Issue:

Running Kubuntu 12.04 with a dual monitor setup - /etc/X11/xorg.conf
shown later. Primary monitor 1680 x 1050 on the right and secondary
monitor 1280 x 1080 on the left

Installed VirtualBox 4.1.8 - 4.1.8-dfsg-2 - installed from Muon.

When VirtualBox is running on the primary monitor and is maximised (via
HOST+F), the following issues arise:

1. Any applications shown on the secondary monitor's Panel via Task
Manager widget cannot be minimised by clicking on the applications tab.
Existing minimised applications on this monitor can be restored by
clicking on their respective tab but subsequent clicks on the tab does
not re-minimise the application. Applications on the secondary monitor
can only by minimised by clicking on Task Manager tabs when VirtualBox
is not in Full Screen mode (VB Host + F)

2. Kickoff Application Launcher menu (present on the primary monitor
only) is rendered under the VirtualBox VM when the VM is in Full Screen.
A keyboard shortcut is assigned to display the Kickoff menu. The menu
renders fine when VirtualBox VM is not maximised. When the VM is in
fullscreen, Kickoff keyboard shortcut does not display the menu - it is
only briefly visible when the Kickoff shortcut menu renders its
slidedown closing animation. Kickoff shortcut is executed outside of the
VB VM by clicking on an empty space on the secondary monitor

3. Applications cannot be dragged from the secondary monitor to the
primary monitor whilst a VirtualBox VM is in fullscreen mode on the
primary monitor. Applications are rendered under the fullscreen VM.

Expected Behaviour:

1. Application can be minimised/maximised on the secondary monitor
irrespective of a full screen application running on the primary monitor

2. Kickoff menu to render above all content on the primary monitor when
kickoff keyboard shortcut is invoked.

3. Dragged application windows from the secondary monitor should render
on top of the active application window on the primary monitor.

:$ lsb_release -rd
Description: Ubuntu precise (development branch)
Release: 12.04

~$ apt-cache policy kde-window-manager
kde-window-manager:
Installed: 4:4.8.1-0ubuntu2
Candidate: 4:4.8.1-0ubuntu2
Version table:
*** 4:4.8.1-0ubuntu2 0
500 http://gb.archive.ubuntu.com/ubuntu/ precise/main i386 Packages
100 /var/lib/dpkg/status

$ apt-cache policy virtualbox-qt
virtualbox-qt:
Installed: 4.1.8-dfsg-2
Candidate: 4.1.8-dfsg-2
Version table:
*** 4.1.8-dfsg-2 0
500 http://gb.archive.ubuntu.com/ubuntu/ precise/universe i386 Packages
100 /var/lib/dpkg/status


/etc/X11/xorg.conf:
# nvidia-settings: X configuration file generated by nvidia-settings
# nvidia-settings:  version 295.20  (buildd@rothera)  Sat Feb 18 09:49:17 UTC 2012

Section "ServerLayout"
   Identifier     "Layout0"
   Screen      0  "Screen0" 0 0
   InputDevice    "Keyboard0" "CoreKeyboard"
   InputDevice    "Mouse0" "CorePointer"
   Option         "Xinerama" "0"
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"
   # HorizSync source: edid, VertRefresh source: edid
   Identifier     "Monitor0"
   VendorName     "Unknown"
   ModelName      "AOC LM720/LM720A"
   HorizSync       30.0 - 83.0
   VertRefresh     55.0 - 75.0
   Option         "DPMS"
EndSection

Section "Device"
   Identifier     "Device0"
   Driver         "nvidia"
   VendorName     "NVIDIA Corporation"
   BoardName      "GeForce 8800 GTS"
EndSection

Section "Screen"
   Identifier     "Screen0"
   Device         "Device0"
   Monitor        "Monitor0"
   DefaultDepth    24
   Option         "TwinView" "1"
   Option         "TwinViewXineramaInfoOrder" "DFP-1"
   Option         "metamodes" "CRT: 1280x1024_75 +0+0, DFP: 1680x1050_60 +1280+0"
   SubSection     "Display"
       Depth       24
   EndSubSection
EndSection