Thanks for making this and sharing it on HN; I like the simplicity of it! The way that selection works really confused me, though. Here's what happened:

I made two objects. I clicked on one, which clearly indicated it as selected. I tried to deselect that object by clicking on the grid outside it, but nothing happened. OK, fine, let's try a different strategy -- selecting the second object (thereby deselecting the first) by clicking on it.

After clicking the second object, it went back to looking like neither was selected. Huh? So I clicked again on the first object to select it... and the second object got selected?! Then I realized that both objects had been selected! I found this behavior so odd that I came here to write this feedback.

FWIW, here's how I think it should work: clicking an item should deselect all the other items unless you're holding the shift key. Selected items should be clearly marked -- don't just dim the deselected items: this keeps the "all selected" and "none selected" states from looking so similar.