The second thing I notice is that when I click, if the click happens to be in the middle bottom of the touchpad, it pastes whatever the last text I selected in any application was. Not from the regular clipboard. The old X11 clipboard I guess. I'd forgotten how weird that thing was.
The secret to this turns out to be that in the scrollpad bar, it asks about the right-click behavior in two places. "Right click in bottom right" also enables a secret mystery middle click. But it doesn't *say* this