By "Private activity", I means activity with history saving disabled. As a user, if I set an activity as private, I don't expect to find an history of what I copy/paste in this activity. I don't really have a solution and I don't know what is technically possible so I just exposes the fact. Maybe Klipper should be disabled on a private activity or provide an option to do it ? Or the history should be cleared when the user switch to an other activity ?
You are right. The private activities should be more integrated with the rest of the system.
The "Clipboard history size" setting could be forced to zero when in a private Activity.
See also bug 427724 for a manual control to temporarily disable saving the clipboard history.