Chrome OS uses "Natural Scrolling", just like the Macs I've been using, so when I switch into xfce and it wants to scroll the old way, I get confused. Thanks to a helpful blog post on enabling natural scrolling in Linux, I didn't even have to think about how to do it. All you need is a line like the following in your xmodmap config:
pointer = 1 2 3 5 4 7 6 8 9 10 11 12
If you want a (slightly) more detailed explanation, just click on the link above. This one wasn't hard to find, but for completeness, I figured I'd document it here.