As I've described in fedora bug https://bugzilla.redhat.com/show_bug.cgi?id=1046207 -- somehow, my mouse config file ~.config/xfce4/xfconf/xfce-perchannel-xml/pointers.xml (supposedly set by xfce4-mouse-settings) had overridden my settings in xorg.conf.d/*.conf
Specifically, I had set it to map double-finger tap to a mouse middle click, but the user config was overriding it. To make things worse, there is no GUI config for this option anywhere, and user is left without an obvious solution.

Please add the missing GUI options that correspond to values from the config file -- specifically, Synaptics_Tap_Action