Backwards Natural Horizontal Scrolling

On a Thinkpad laptop with Ubuntu 20, I switched to Natural Scrolling, but was confused to find that, although vertical scrolling worked as expected, horizontal scrolling was unchanged, resulting in a very unnatural experience. It turns out that the culprit was a old input driver that was probably carried along as this laptop was upgraded over the years. The correct driver (xserver-xorg-input-libinput) was already installed, so the fix was to uninstall the old one:

sudo apt remove xserver-xorg-input-synaptics

then log out and back in again. Touchpad speed was at zero, so that had to be moved to 2/3 of maximum to restore the speed I had under the Synaptics driver.