After a recent mesa update on gentoo-hardened, Firefox context menu
fades in with artifacts, as if fade-in effect was drawn too slowly. It's
not a big deal but it's certainly visible.

I can't be absolutely sure it's mesa. Looks like that.

Info from eix output:

     Installed versions:  17.1.10^d(01:53:11 2017-12-05)(classic dri3 egl gallium gbm nptl pax_kernel pic vaapi -bindist -d3d9 -debug -gles1 -gles2 -llvm -opencl -openmax -osmesa -selinux -unwind -valgrind -vdpau -vulkan -wayland -xa -xvmc ABI_MIPS="-n32 -n64 -o32" ABI_PPC="-32 -64" ABI_S390="-32 -64" ABI_X86="64 -32 -x32" VIDEO_CARDS="i965 r600 -freedreno -i915 -imx -intel -nouveau -r100 -r200 -r300 -radeon -radeonsi -vc4 -vivante -vmware")

No other rendering issues were observed. However, I don't use demanding
graphics all that much; it has always been Firefox that produced
artifacts.

Not sure if this can be helped easily, unless there's something with my
USE flags that more experienced mailing list readers would find
suspicious.