At some point of time for unknown reasons pidgin package stopped inheriting gnome2.eclass with a comment of # mimicking gnome2_src_install as that one is banned for >=EAPI-6 (*sigh*) (which I don't think to be true since years, if ever). Ever since then, pidgin has failed to do the xdg-utils and gnome2-utils dance of updating various caches; so for example a freshly installed pidgin doesn't show an application icon in gnome-shell until something else is installed/upgraded/removed that happens to properly call gnome2_icon_cache_update from gnome2-utils.eclass in postinst and postrm.
commit 42f17a30c10f3bb5a3a84c51b770aa7262919d85 Author: Lars Wendler <polynomial-c@gentoo.org> Date: Mon Feb 27 22:32:21 2017 net-im/pidgin: Reinstate gnome2 eclass to get rid of an xdg issue. This fixes https://bugs.gentoo.org/611138 Package-Manager: Portage-2.3.3, Repoman-2.3.1