Order of dragging from child to parent can cause future hovers on parents to fail.

https://jsfiddle.net/5dhfmmxf/4/

Drag the green onto the red and the red turns purple as expected with the hover class. Drag the blue onto the green parent. Now try to drag the green onto the red again and no purple hover class is added.

To reset it so it works again drag the blue onto the red and it will reset whatever state there is so you can drag the green onto the red again.

Also I think this should be marked important. I noticed this error has confused people for years but they couldn't reproduce it or didn't submit a bug. I think it causes a number of complex hover issues in larger codebases that make reproducing it difficult.