I don't know if it is relevant, but my logged in X11 session is vt2, and the gdm login screen is on vt1. i.e. I can ctrl+alt+f1 and it will go to the login screen, and it will then click back to vt2 after re-logging in. This is the 'default' on my system (i.e. I haven't knowingly done anything to make it this way). I wonder if this could be something that messes with the display context passed through to the X server for pasting. Does anyone else have this same setup?