I've had x11-themes/oxygen-gtk-1.1.4:3 installed for some time already when I upgraded x11-libs/gtk+ from 3.6.x to 3.8.x. After the upgrade, menus in all gtk+ apps started to render incorrectly so I checked if there was already a bug reported to oxygen-gtk. And as it turned out, the bug has been long fixed and the fix was part of the oxygen-gtk version I had installed[1]. But the fix was ifdef'ed to gtk+ versions >= 3.7.0 at _build time_[2]. I checked and there are more such ifdefs in the code. Conclusion: either encourage upstream to remove build-time-version-dependent code and not write such in the future, or force rebuild of x11-themes/oxygen-gtk:3 in Gentoo after each x11-libs/gtk+:3 upgrade (minor version upgrades will probably suffice). [1] https://bugs.kde.org/show_bug.cgi?id=312988 [2] https://projects.kde.org/projects/playground/artwork/oxygen-gtk/repository/revisions/f386f3a9c853dd1d6009f0e5bf3e3fc36231ddd6/diff
There really isn't anything we can do here; I don't believe Portage can force rebuilds on minor version bumps (and subslots aren't appropriate here). Your best bet is to file a bug with oxygen-gtk upstream.