Comment 41 for bug 1368402

Running Xubuntu 18.04LTS, XFCE 4.12

I have the same issue. and the solution provided by https://bugzilla.xfce.org/show_bug.cgi?id=11193#c8 (comment 8) does not fix the problem for GTK3 and other apps. the technique updated by comment 14 does not fix this either. the fix does not reverse the direction of scroll in apps. but it does reset/reverse-reverse them for the ones in which the reverse scroll was working.

to disable the "fix" I did xinput --set-int-prop 8 "libinput Natural Scrolling Disabled" 8 1 and the system went back to the original state with reverse scroll for xfce apps working and not working for other apps

xinput device properties
Device 'SynPS/2 Synaptics TouchPad':
 Device Enabled (143): 1
 Coordinate Transformation Matrix (145): 1.000000, 0.000000, 0.000000, 0.000000, 1.000000, 0.000000, 0.000000, 0.000000, 1.000000
 Device Accel Profile (276): 1
 Device Accel Constant Deceleration (277): 2.500000
 Device Accel Adaptive Deceleration (278): 1.000000
 Device Accel Velocity Scaling (279): 12.500000
 Synaptics Edges (299): 1618, 5366, 1356, 4536
 Synaptics Finger (300): 25, 30, 0
 Synaptics Tap Time (301): 180
 Synaptics Tap Move (302): 251
 Synaptics Tap Durations (303): 180, 180, 100
 Synaptics ClickPad (304): 0
 Synaptics Middle Button Timeout (305): 75
 Synaptics Two-Finger Pressure (306): 282
 Synaptics Two-Finger Width (307): 7
 Synaptics Scrolling Distance (308): 114, 114
 Synaptics Edge Scrolling (309): 0, 0, 0
 Synaptics Two-Finger Scrolling (310): 1, 1
 Synaptics Move Speed (311): 1.000000, 1.750000, 0.035014, 0.000000
 Synaptics Off (312): 1
 Synaptics Locked Drags (313): 0
 Synaptics Locked Drags Timeout (314): 5000
 Synaptics Tap Action (315): 2, 3, 0, 0, 1, 3, 2
 Synaptics Click Action (316): 1, 1, 0
 Synaptics Circular Scrolling (317): 0
 Synaptics Circular Scrolling Distance (318): 0.100000
 Synaptics Circular Scrolling Trigger (319): 0
 Synaptics Circular Pad (320): 0
 Synaptics Palm Detection (321): 0
 Synaptics Palm Dimensions (322): 10, 200
 Synaptics Coasting Speed (323): 20.000000, 50.000000
 Synaptics Pressure Motion (324): 30, 160
 Synaptics Pressure Motion Factor (325): 1.000000, 1.000000
 Synaptics Resolution Detect (326): 1
 Synaptics Grab Event Device (327): 0
 Synaptics Gestures (328): 1
 Synaptics Capabilities (329): 1, 0, 1, 1, 1, 1, 1
 Synaptics Pad Resolution (330): 76, 44
 Synaptics Area (331): 0, 0, 0, 0
 Synaptics Noise Cancellation (332): 28, 28
 Device Product ID (269): 2, 7
 Device Node (268): "/dev/input/event6"
 libinput Natural Scrolling Enabled (280):

Apps affected: (no reverse touchpad scroll)
Okular
 Evince
 Qpdfview
 libreoffice writer
chrome (pdf view specifically)
xfce4-terminal ( this was a surprise)

Not affected:
Thunar
PCMan file manager