How to get stacking layout in Regolith Linux?

I just installed Regolith on top of a normal Ubuntu installation. After some conflicting advice between the on-screen help and wiki (Super+t, not Super+Backspace) I managed to toggle between vertical, horizontal and tabbed layouts. It skips the other standard layout option in i3wm, which is stacking. How do I get a stacking layout in Regolith?