But stable doesn't use this GTK3 version, so it's not there (or not as much).
This is how mouse scrolling works in Settings Manager in current Xfce version:
yes = mouse scroll enabled
no = no scroll, options hidden
n/a = no scroll, but options visible without scrolling
ext = opens external window
Appearance style yes icons yes fonts n/a settings n/a Desktop background yes menus n/a icons n/a default icons yes File Manager ext Notifications general n/a applications no log n/a (but 2nd outer scrollbar needed to see buttons at bottom) Panel display n/a appearance n/a items yes Preferred Applications n/a Window Manager style:theme yes keyboard yes focus n/a advanced n/a Window Manager Tweaks n/a Workspaces n/a Xfce Terminal ext Display n/a Keyboard behavior n/a application shortcuts yes layout n/a Mouse and keyboard n/a Power Manager n/a Removable drives and media n/a Accessibility n/a Add/Remove Software ext Firewall ext LightDM GTK+ Greeter appearance n/a panel n/a window position n/a misc n/a Manjaro Notifier Settings ext Manjaro Settings Manager ext MIME ext Session and Startup general n/a splash n/a application autostart yes session yes advanced n/a Software Update ext Bluetooth Adapters (no open but i have no bluetooth) Settings Editor yes
Summary: In the current Xfce version, the only Settings Manager menus that has a scroll problem (usability not function) is Notifications:Applications and Notifications:Log.