If you open a floating library window and drag several books from it, one by one, into the "Plus these resources" section of a Collection definition, every time you drop a book, the Library window scrolls back up to the top, so you have to scroll down again to find where you were. This is very inconvenient if you're trying to add several books in a row.
Furthermore, it would be really nice (and would make the above problem basically moot) if you could select several books in the Library that you want to add to the "Plus these resources" section and drag them all at once. But if you do a multi-book select, dragging only drags the one you clicked on.