Steps to reproduce:
0. enable autoscrolling

1. open the attached testcase
2. middle-click in zebra strips

Expected result:
you should be able to scroll both horizontally and vertically, like when you use wheel to scroll

Actual result:
you can only scroll horizontally because the innermost container is only horizontally scrollable

This is annoying on certain website, for example on GitHub viewing code like this, because the code area is horizontally scrollable, if you trigger autoscrolling from that area, you can't scroll along the code vertically to browse the whole file.

https://searchfox.org/mozilla-central/rev/d2cec90777d573585f8477d5170892e5dcdfb0ab/toolkit/actors/AutoScrollChild.jsm#158 only cares about closest scrollable element... it should probably keep one element per scrollable direction or something.

Note that you linked to the main-thread implementation of autoscrolling. APZ has another implementation which presumably gets used more often. If we want to implement scroll handoff there (which is kind of what this amounts to, although perhaps with some modifications) we'd want to make changes around here.

It looks like bug 1437019 has been open for this for a while, let's dupe this one over.