Well, I had this little problem today, and I also noticed that there were a lot of other people out there that didn’t understand why it was happening. Which is why I decided I should post the solution.
The problem of not being able to drag elements in to empty lists, only occurs when the empty list doesn’t have any dimensions… Okay, that sounds confusing. Take my example.
If I have a list, and the list element’s padding and margin have been set to 0px with css – try to view the list element’s area on the screen using a tool like firebug, you’ll notice that it basically isn’t there.
So, technically, the drag and drop between the lists doesn’t work because there’s no longer (or never was if the list is initially empty) any element to drop the new child element IN to.
The solution? Well of course, just add some padding in the css to your UL or OL, and you’ll see that it has some substance regardless of whether it has child list items or not. You’ll then be able to drag in to the empty list perfectly.
Another solution, which might not be as reliable but it’s still good, is to have a min-height css attribute for that list element, so regardless of your lists’s number of list items, it has a certain element height at all times.
Easy peasy. I’ll bet the same solution can be used for Scriptaculous and other scripting technologies.