update function in order to check, enforce, and manipulate the DOM based on whatever restrictions you’d like.
However, now in version 4.1 there is another way of inserting widgets into widget areas, that don't involve the Sortable library. Now there's the
widgets-chooser that opens when clicking a widget, and you get a list of all areas into which you can insert the widget.
I'd like to disable that list, or, alternatively, hook onto its insert function and run the same kind of code that prevents multi widgets in the same area.