This is a can of worms... There is much discussion on this. You might start looking at: https://bugzilla.gnome.org/show_bug.cgi?id=333514 or http://webcvs.freedesktop.org/icccm-extensions/selections/clipboards.txt?revision=1.3&view=markup

Typically, the applications I use which are "traditional X11 apps" have behavior #4, however, I would find behavior #3 most useful (but is sadly, not used by most apps).

Good Day,
    Dean

In reply to Re: X11 pasting by duelafn
in thread X11 pasting by Sewi

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