The brightness control slider of xfce4-power-manager panel plugin (version xfce4-power-manager-1.6.0.41.g9daecb5-1-x86_64.pkg.tar.xz on Antergos) doesn't work with gtk-3.22.18-1 Clicking on the slider knob itself makes the plugin window disappear, clicking anywhere else on the slider sets the brightness to 0 and the screen goes black. Roll back from gtk-3.22.18-1 to gtk-3.22.17-1 fixed it.
This problem is not only with brightness slider in xfce4-power-manager. Sliders in xfce4-pulseaudio-plugin (on Arch) have the same issue. It's something related with gtk3-3.22.18-1 update. Also i checked updating to gtk3-3.22.18-2 but problem still persist.
*** Bug 13769 has been marked as a duplicate of this bug. ***
This is a regression in Gtk+ introduced here: https://git.gnome.org/browse/gtk+/commit/?h=gtk-3-22&id=9b032073cbcf5aadbeb4b74f0cf662a9fa5f5530 and was fixed in gtk+3.0 - 3.22.19 See also: https://bugzilla.gnome.org/show_bug.cgi?id=786029