Impressive!

Two comments:

- Using f1 on chrome brings up the Chrome help page. You might want to make that another button.

- It took me a while to realize that you're supposed to drag objects from the left, and not use the mouse to define where they should go. That lead me to accidentally select or move existing objects while attempting to place new objects with my mouse. I'm not sure how common my experience was and I'm not sure how to address it (or even that you should). But I figured you should still keep it in mind.