Focus is shifted to "random" window, not to desktop

The following situation:
Have one application window open, e.g., thunderbird. Double click a file on the desktop (e.g., a picture) to view it. Close this new window. Press delete to delete the picture on the desktop.

Expected outcome: Desktop has focus (as it had the focus previously, when clicking on the desktop) and the picture is deleted from the desktop.
Actual outcome: The last selected window (e.g., thunderbird) has the focus and you delete an e-mail.

This is very annoying, as I accidentally deleted e-mails in thunderbird when I wanted to clean up my desktop.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: unity 5.16.0-0ubuntu1
ProcVersionSignature: Ubuntu 3.2.0-32.51-generic 3.2.30
Uname: Linux 3.2.0-32-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.0.1-0ubuntu13
Architecture: amd64
CompizPlugins: [core,composite,opengl,compiztoolbox,decor,vpswitch,snap,mousepoll,resize,place,move,wall,grid,regex,imgpng,session,gnomecompat,animation,fade,unitymtgrabhandles,workarounds,scale,expo,ezoom,unityshell]
Date: Mon Oct 15 11:11:20 2012
EcryptfsInUse: Yes
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Beta amd64 (20120301)
SourcePackage: unity
UpgradeStatus: No upgrade log present (probably fresh install)