Make three-finger tap middle-click again in Ubuntu/Xubuntu 18.04 LTS

Perhaps this also affected earlier versions, such as 17.04 or 17.10, but I'm only experiencing this now. After installing 18.04 LTS, my ThinkPad's touchpad no longer registers a three-finger tap as a middle click. In fact, a three-finger tap now does nothing.

It's easy to fix this. First, let's make a xorg.conf.d directory, if you don't already have it:

$ sudo mkdir -p /etc/X11/xorg.conf.d
$

Now you're going to use your favourite editor, and edit /etc/X11/xorg.conf.d/70-synaptics.conf and put the following inside it:

Section "InputClass"
    Identifier "Touchpad"
    MatchIsTouchpad "on"
    Driver "synaptics"
    Option "TapButton3" "2"
EndSection

Now reboot, and you're done. Three-finger tap should be middle-click.