To see the error do the following:

1. yarn && yarn start
2. Go to localhost:8080
3. Open the console (so you can see the errors
4. Drag one of the list items on top of another list item.

You should see an error stating:

Uncaught Error: Cannot call endDrag while not dragging.

For contrast, in a different terminal window, run yarn webpack:dev. This is a slightly different config, but the key seems to be the lack of minification.

Go back to the browser, and drag again. You should see console messages for hovering and dropping.