Ok, so I just tried out yabai. While this looks like a great WM, unfortunately it does not really support i3 stacking and tabbing features — at least for the moment (see also this more recent update), which (I believe) would be the most interesting options for small Retina displays. Update: I reinstalled the HEAD version and tried the “stack” layout, which looks like a simple fullscreen display, not really stacking à la i3wm.