[ubuntu] Strange behaviour when minimizing virtualbox in fullscreen mode

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http://ubuntuforums.org – When virtualbox is running in fullscreen mode, and I'll minimize it to go back to Unity, the only visible elements are the wallpaper and the background shadow of the upper bar. If then I click on virtualbox main window return all normal. I want to report a bug, but I don't know if is Unity or Virtualbox issue. Thanks in advance! (Hardware)