After installing Ubuntu, Kubuntu, Edubuntu, etc. in parallel on my laptop, my Opera browser started misbehaving when logged in using the Ubuntu desktop environment. There was a problem with rendering, including the closing of tabs leaving ghostly after-images, etc.However, it worked fine when using the KDE Plasma environment in Kubuntu. It took me a long time to work out how to fix it. Basically, the problem is related to conflicts between gtk and kde applications, and the correct theme must be set in the desktop environment. I found the solution on this website:

http://www.gefoo.org/generalfoo/2012/03/25/gtk-using-ugly-theme-in-kde-4-8-1-on-ubuntu-11-10-oneiric/

I ran the following commands to install the gtk-theme-switch application and changed the theme to the Ubuntu 13.04 default theme, Adwaita. Suddenly, my Opera browser was working fine again!

sudo apt-get install gtk-theme-switch gtk2-engines-oxygen
gtk-theme-switch
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