Provide user feedback when trying to drop in a prohibited placeholder

[aximuseng]

For some more background:

https://stackoverflow.com/questions/64436263/jquery-nestable2-adding-placeholder-css-feedback

I already have a beforeDragStop call that does the logic to allow / dis-allow the dropping of an element in the placeholder. I want to now change the CSS of the placeholder also (to red etc) for user feedback.

I can't seem to find in the code where adding the placeholder to the DOM is clearly triggered. If I knew that I think I could easily add this.