On my system with LXDE installed, 'emerge -auDN @world' resulted in a stalemate condition:

libfm-1.2.3 is hard blocking libfm-extra-1.2.3, however libfm-1.2.3 is slated to be upgraded to libfm-1.2.3-r1, which would remove the block. libfm-1.2.3-r1 cannot be installed because it requires in libfm-extra-1.2.3.

I fixed this by running 'emerge --unmerge libfm' followed by 'emerge libfm libfm-extra'.

It seems to me that this should have been handled automatically. If it can't get fixed to be handled automatically, then at least the solution is documented here.