As a Roblox developer, it is currently impossible to use a scroll wheel to scroll on a horizontal frame.
I’m not sure if this ever happened or if it is a bug, but I’m unable to use my scroll wheel to scroll on a horizontal scrolling frame.
It feel so counter intuitive to have the scroll wheel work on one frame, and then not work on another frame which is identical except for the scrolling direction.
Somebody posted about this early last year, but there was only a hacky workaround.
EDIT: Thanks to @Dynamese it turns out that sideways scrolling with the scroll wheel only works if you are holding down shift at the same time. This can be rather inconvenient if you have any abilities in the game bound to shift.
Repro: XAxisScrollingFrameRepro.rbxl (15.5 KB)