It was this one; the current layout still doesn't enforce the intended solution properly but maybe that's ok.
Ah. Turns out I'd actually not made it past the level I posted, but I'm on 13 now. I wonder what the intended solve is? It's pretty interesting that basically all the puzzles in this one rely on overlaps.
Yeah, overlapping ended up being pretty much the most interesting element of the mechanic. Level 13 was meant to force you to overlap all 3 boxes.