lightdm was cited as too old when I filed a bug about excessive memory usage. Just took a look at the ebuilds and noticed that stabilization has lagged. Reproducible: Always
Shortly using lightdm-1.8.5 and lightdm-gtk-greeter-1.6.1 here in amd64 stable and so far it works fine. Certailny better than current stable lightdm (1.4.0-r2) which even does not read its configuration file /etc/lightdm/lightdm.conf and wants to pull testing dev-libs/gobject-introspection-common-1.34.2 and dev-libs/gobject-introspection-1.34.2-r1.
lets do it =x11-misc/lightdm-1.8.5 =x11-misc/lightdm-gtk-greeter-1.6.1
amd64/x86 stable
arm stable, all arches done.