Oh, this is cool. My favorite part is being able to mouse over the health bars at the right, and see exactly which bits they draw from in the code itself.
Draw on a QR code, and the health bars will accurately display how close the QR code is to being unscannable. How few errors does it take to break a QR code? How many errors can a QR code handle? Counters at the bottom track your record minimum and maximum damage. (Can you figure out how to break a QR code with 0.0% damage to the actual data region?)
Also, click on the magnifying glass button to toggle between "draw mode" and "inspect mode". I encourage you to use your phone's camera to scan the code as you draw and undo/redo to verify that the code really does break when the app says it does.
I wrote the underlying decoder in C++, and it's compiled to WebAssembly for the website.
I hope you find it interesting.