Really a task ticket. I'm not sure the existing validation is strict enough on these cases.
The entry on my paper to-do list is “Validate whether the user can drag from a chapter”. This is followed on the paper to-do list by “YES if the drag would not empty the chapter or it is the last chapter”, but both of these criteria are stricken out for being nonsense. (See #3 for what we should do when emptying a chapter.)