Simon, I'm working on a Mac and they have quite strict style guides which most people take serous. I know that in the Windoze world this is taken quite sloppy.
I do not regularly use mac since 10.4, but at the time I felt all serious consideration of User Interface guideline ended with os 8, and UI became really incoherent with OS X. My later encounters with macs just reconfirmed my choice to stay away from that UI.
Anyhow, using mostly Windows and Linux, I can pretty much in every application rely on the following d&d behavior in hierarchical lists:
- when you drag over an item that can contain other items (folder), the item gets highlighted to indicate that; when you drop in that position, the load goes inside the target item.
- when you drag over an item that can not contain others, or on the margin of a folder, the expected insert position is indicated as a line after the target item, whil target item is not highlighted; when you drop, the load is inserted after the target item.
You can check this behavior, for example, on firefox on any platform (other than mac) when dragging bookmarks in bookmarks menu. On mac I believe xcode has the described behavior for project files list.