Drag&Drop in a QTreeView behaves strange on Mac OS X

The problem shows up in QtCreator. There we have a Navigator that is a (heavily styled) QTreeView. You can use it to reorder the Item hierarchy. While this works nicely on Windows/Linux, it shows severe problems on Mac OS X / Cocoa:

• While dragging, we leave artifacts behind (the dragged item just remains visible at old positions)
• Sometimes you cannot drag any more to positions you should be able to drag. E.g. dragging to the root item often doesn't work (the mouse cursor is turned to default, no marking of the item to drag to). But then again, sometimes it works ...

Attached is a version of the navigator ripped out of Qt Creator. This version shows also the second problem, but (for whatever reason) doesn't show the artifacts.