There's a bug that I seem to have triggered because it's possible to space the pieces unevenly. When I place a new piece that can't align with one of them unless it overlaps the other, that new piece sort of cycles back and forth between a couple of positions without ever settling (which brings the game to a halt). Don't know whether that's the same bug as the "exclusive to HTML5 export" one you mention.
I really appreciate the feedback. Im starting work on overhauling the entire game system on the back end now. For some reason I thought it was a good idea to use the physics engine to make things fall and detect certain types of collisions, and that seems to be the root of all the problems. Going to essentially recode it, so check back here in a week or two