Comment 16 for bug 11334

Ubuntu QA guy Simon Law puts it like this:

"Thank you for your report.

This behaviour has always been the case and is a design feature of X11.

You are describing SELECTION and PRIMARY buffers. They were separate and have semantics that are both supported by major X applications. It is unlikely that this will change in the near future, even though more and more applications prefer exposing the PRIMARY buffer in their menu systems."