Hmm, my guess is you're trying to use the box's borders as lines (they don't count, only the lines you draw count). Let me know if that's not the issue. Also, I'll think about ways to make this more clear!
Entirely my mistake because I was kind of ignoring everything you said lol
Alternatively, you could automatically "close" figures after they have at least three points, and give a hint of a handle that allows dragging a new point out of the middle of an existing side.