These packages make use of gtk3 useflag which indicates that there are issues of migration to gtk+:3.
Gtk3 and similar useflags should be avoided.
Adding to tracker. Please have a look.
Both ebuilds install a dynamically loaded GTK+ 3 compatible library, or they do not when USE=-gtk3. When, say, GNOME is detected, this library takes care of displaying file dialogs and such that match the desktop environment, unless autodetection is turned off in favour of another specific implementation.
Same for KDE 4 and GTK+ 2 (as well as a generic "X11" interface).
All of this is closed source and nothing is actually compiled against the installed base.
Do you suggest changing IUSE=gtk3 to IUSE=gtk?
Yes, imo it makes sense to merge both flags for these ebuilds. We generally should expect the user to have gtk+3 anyway, cause it is the latest and stable version.
As far as I understand this does not even break anything.
Fixed in =www-client/opera-12.00.1456 and =www-client/opera-next-12.00.1456. The older ebuilds will disappear soon anyway.
USE=gtk3 is gone.