libinput -- disable horizontal scrolling

The Logitech Trackman does not have a scroll wheel so we enable it by installing libinput and this libinput.conf Section "InputClass" Identifier "Marble Mouse" MatchProduct "Logitech USB Trackball" Driver "libinput" Option "ScrollMethod" "button" Option "ScrollButton" "8" Option "MiddleEmulation" "true" EndSection This works well. Combining small-left button with a vertical ball roll scrolls the page vertically but if there is an active horizontal scroll bar the page jitters. A perfectly straight roll requires precise movement and I never need a horizontal scroll. The libinput documentation has this to say Horizontal scrolling Scroll movements provide vertical and horizontal directions, each scroll event contains both directions where applicable, see libinput_event_pointer_get_axis_value(). libinput does not provide separate toggles to enable or disable horizontal scrolling. Instead, horizontal scrolling is always enabled. This is intentional, libinput does not have enough context to know when horizontal scrolling is appropriate for a given widget. The task of filtering horizontal movements is up to the caller. Here's the rub: **libinput does not provide separate toggles to enable or disable horizontal scrolling.... The task of filtering horizontal movements is up to the caller.** Who is the 'caller' here and how could I configure the caller to drop horizontal movements.