fully up to date ~amd64 system breaks multi monitor capability when upgrading to xfce-base/xfce4-settings-4.13.6 The respective display settings dialog becomes empty and the system defaults to mirroring even on previously working multi monitor (extending) setup. Probably this is an upstream bug, but I cannot tell. Downgrading to 4.13.5 makes it work again.
It is sad to read that you have problems with the software. The situation seems to be a bit more complicate and requires some analysis. We can not help you efficiently via bug tracker. The bug tracker aims rather on specific problems in .ebuilds and less on individual systems. I have had very good experience on the gentoo IRC [1] with questions like this. Of course there are also forums and mailing lists [2,3]. I hope you understand, that I will close the bug here therefore and wish you good luck on one of the mentioned channels [4]. Please reopen the ticket in order to provide an indication for an specific error in an ebuild or any gentoo related product. [1] https://www.gentoo.org/get-involved/irc-channels/ [2] https://forums.gentoo.org/ [3] https://www.gentoo.org/get-involved/mailing-lists/all-lists.html [4] https://www.gentoo.org/support/