Another idea mentions bringing this capability to a certain type of mouse but I'd like to see horizontal scrolling work for every trackpad and every type of mouse that has a side scrolling mechanism. Not all mice come with the software mentioned in the workaround, and trackpads are surely out of luck. Trackpads are already quite difficult to use in GIS; this is just insult to injury. Side scroll works fine in ArcMap/ArcCatalog and nearly every other Windows program; going back and forth between Pro and everything else is a little annoying to say the least. Hoping Esri will get with the times on this!