Scrolling with mouse fails in some plugins. Apparently "This is a regression in xfce4-settings and unrelated to xfce4-notifyd - it affects all other plugged Gtk3 dialogs as well as far as I can tell." https://bugzilla.xfce.org/show_bug.cgi?id=13275#c5
Yes, it affects any scrolled window of plugged gtk3 dialogs. The workaround I found is to click (left or right) and hold on any part of the scrolled window then mouse wheel works. After the mouse button is released, it'll briefly work. I suspect it's not an issue about events passage, but related to focus.
Scrolling works if you click on the mouse wheel and keep scrolling it
*** Bug 14732 has been marked as a duplicate of this bug. ***
Another workaround: $ GDK_CORE_DEVICE_EVENTS=1 xfce4-settings-manager
Fix applied with the below commit. https://git.xfce.org/xfce/xfce4-settings/commit/?id=b664ea04a235051fb27748bd01a6b7823d671e8b
*** Bug 13317 has been marked as a duplicate of this bug. ***
*** Bug 14879 has been marked as a duplicate of this bug. ***