xfce4-lipman question

[Manfred Lotz]

Hi there,
I tried out xfce4-clipman and noticed a strange behavior.

I go to a github repository. Then click on 'Click or Download" and in
the small window popping up I click the symbol to get the URL into the
clipboard.

The good:

- Now, the URL is in the clipboard
- In a libreoffice document I type Ctrl-C to paste the URL into the
  document which works fine

The bad:
- Clicking the mouse's middle-button and nothing happens.


How can I make the mouse's 'middle-button' function? 

I should add that if I mark text with the mouse I easily can paste
using the middle-button.

-- 
Manfred