I have installed the current git version of XFCE under Arch, specifically to test the new display settings options but unfortunately it fails to enable my external 4K HDMI display. My display is shown when attached within the main XFCE display settings window but it is only ever Disabled. When I click on the toggle switch to the right of the display list dropdown menu, the "Would you like to keep these settings?" prompt appears instantly but my HDMI display doesn't shown anything. I click "Keep this configuration" and at this point I can still see my HDMI display in the graphical rep. of the displays but it is still labelled as disabled when the toggle says it should be enabled. I am able to enable this same display fine under the stable XFCE release or by using MATE or xrandr etc.
$ xfce4-display-settings (xfce4-display-settings:16488): xfconf-WARNING **: 22:44:52.727: Error check failed at xfconf_channel_get_internal():455: GDBus.Error:org.xfce.Xfconf.Error.PropertyNotFound: Property "/IdentityPopups" does not exist on channel "displays" (xfce4-display-settings:16488): xfconf-WARNING **: 22:44:52.728: Error check failed at IA__xfconf_channel_has_property():612: GDBus.Error:org.xfce.Xfconf.Error.PropertyNotFound: Property "/panels/panel-0/output-name" does not exist on channel "xfce4-panel" (xfce4-display-settings:16488): xfconf-WARNING **: 22:44:52.729: Error check failed at xfconf_channel_get_internal():455: GDBus.Error:org.xfce.Xfconf.Error.PropertyNotFound: Property "/desktop-icons/primary" does not exist on channel "xfce4-desktop" (xfce4-display-settings:16488): xfconf-WARNING **: 22:44:52.730: Error check failed at xfconf_channel_get_internal():455: GDBus.Error:org.xfce.Xfconf.Error.PropertyNotFound: Property "/primary-monitor" does not exist on channel "xfce4-notifyd" (xfce4-display-settings:16488): xfconf-WARNING **: 22:44:52.730: Error check failed at xfconf_channel_get_internal():455: GDBus.Error:org.xfce.Xfconf.Error.PropertyNotFound: Property "/AutoEnableProfiles" does not exist on channel "displays" (xfce4-display-settings:16488): xfconf-WARNING **: 22:44:52.733: Error check failed at xfconf_channel_get_internal():455: GDBus.Error:org.xfce.Xfconf.Error.PropertyNotFound: Property "/Schemes/eDP1/EDID" does not exist on channel "displays" (xfce4-display-settings:16488): xfconf-WARNING **: 22:44:52.733: Error check failed at xfconf_channel_get_internal():455: GDBus.Error:org.xfce.Xfconf.Error.PropertyNotFound: Property "/Schemes/HDMI1/EDID" does not exist on channel "displays" (xfce4-display-settings:16488): xfconf-WARNING **: 22:44:52.733: Error check failed at xfconf_channel_get_internal():455: GDBus.Error:org.xfce.Xfconf.Error.PropertyNotFound: Property "/Notify/eDP1/EDID" does not exist on channel "displays" (xfce4-display-settings:16488): xfconf-WARNING **: 22:44:52.734: Error check failed at xfconf_channel_get_internal():455: GDBus.Error:org.xfce.Xfconf.Error.PropertyNotFound: Property "/Notify/HDMI1/EDID" does not exist on channel "displays"
This bug can be closed now. The cause of my problems was the xfconf-git AUR package being built without: glib-perl perl-extutils-depends perl-extutils-pkgconfig I have posted a comment on the AUR page for that package requesting that it be updated/fixed: https://aur.archlinux.org/packages/xfconf-git I had to also rebuild xfce4-settings-git after building xfconf-git with those packages installed.