Go to menu at the top of the window:
Actions > Show actions list...
write "horizontally" in the filter, click the action "View: scroll horizontally
(MIDI CC relative/mousewheel)", click "Add..." in the lower left corner, and
swipe left or right with two fingers on the trackpad.
do the same for "vertically" with vertical swipe.
You now have standard OSX scrolling behavior in reaper arranger.
Do the same for MIDI Editor by checking "MIDI Editor" section in the top
right corner of action list.
You can map any mouse gestures and keyboard shortcuts the same way.
BTW, there are annual discussions that this should be the default behavior, at
least for OSX, but since Reaper design was based on Vegas, the scrolling zooms.
Hope it helps
e