This is in connection with https://bugzilla.gnome.org/show_bug.cgi?id=786029 I upgraded to gtk3-3.22.18.1. xfce-panel plugins with a slider - for example, the power-manager plugin (https://docs.xfce.org/xfce/xfce4-power-manager/1.4/panel-plugin) with the brightness slider don't work as intended. When I try dragging the slider, the parent window disappears (loses focus) and the slider value is not changed. I downgraded to gtk3-3.22.17-1 and the slider is fine.
This is unrelated to the power manager, but in fact a regression in Gtk+3. Upstream bug (with working patch) is here: https://bugzilla.gnome.org/show_bug.cgi?id=786029