I just did an in-place upgrade from Ubuntu 16.04.6 LTS to 18.04.2 LTS and have found that a double-tap is seemingly now required to invoke a Tap to Click with my ALPS DualPoint TouchPad device. There does not appear to be a way to change this in the GUI and I have no idea which one of the numerous TouchPad CLI properties might control this. To be clear, a single-tap does not appear to be doing anything, but a single-click of the physical left button invokes a click. So far it seems all other TouchPad behaviors/actions/sensitives are fine, so I am hesitant to just start messing with various CLI values. I've done a bunch of searching and while there are a lot of complaints about TouchPad sensitivity and action changes I haven't found anything talking about this specific symptom.