Our version of gtk+ doesn't conform to the issue in bug 111483. See the gtk+ ChangeLog line 592. Long story short, under pkg_setup() in the gtk+ ebuild: -if use X && ! built_with_use x11-libs/cairo X; then +if ! built_with_use x11-libs/cairo X; then
I think I fixed this. It was not a main tree deviation, but an aqua extension problem.