The current gnome-panel-2.18.3 ebuild has RDEPEND >=x11-libs/libwnck-2.13.5 , but the emerge of gnome-panel-2.18.3 will also fail if x11-libs/libwnck-2.20.1 is installed. Error message: window-menu.c:254: error: too many arguments to function `wnck_selector_new
You're mixing Gnome 2.18 and 2.20 ... I'm not sure I want to bother putting a blocker inside gnome-panel. Thanks for reporting anyhow.
Don't mix Gnome 2.18 and Gnome 2.20. If you run a stable system and get libwnck-2.20.x from unstable, then you either get all of gnome to 2.20 or you are on your own. We won't put in extra blockers that cause major pain and blockers on upgrading from 2.18 to 2.20 when it goes stable. libwnck-2.20 and all things that won't work with libwnck-2.20 (and have an upgrade that works with it) will go stable at the same time. Additionally x11-wm/compiz-0.6.2 is at the same visibility level (~arch) as Gnome-2.20 and the gnome team will not support mixing things like that if you choose to run compiz with a stable tree by marking it stable yourself in package.keywords.
*** Bug 199383 has been marked as a duplicate of this bug. ***
*** Bug 200222 has been marked as a duplicate of this bug. ***