What almut said, there's no "X11 default". All that X11 does is sending the application a mouse-down and a mouse-up event, which in many cases result in the application doing nothing at all. Also note that many application do whatever it would have done if you've typed 'abc' on the keyboard while whatever widget that contains the 'def' has the focus. But other applications do other things: if I select the "abc" in Firefox, and middle-click while hovering over "def", my Firefox loads the page
http://abc.go.com.