I am also getting this strange behavior with kgx (gnome-console), so it
seems related to gnome but not necessarily gnome-terminal in particular.
The problem is even worse with kgx, where shift + right-click both
starts a search for the current word (inside vim) and prints the menu
"[copy - ]paste - select all", whereas gnome-terminal only shows a
terminal menu.
In vim, I have the option mouse=a (the default). I can work around this by
changing this option to disable use of the mouse, but that's a bit
extreme.
--
Marc Glisse