The showFeedback method of the DragManager doesn't seem to do
anything when dragging existing elements in an AIR app. It DOES
however work when dragging from outside to inside the AIR app.
In my example below, i've tried to make it very simple,
basically, i've got a tree that represents files and folders. You
should ONLY be able to drag onto folders, not files. If you drag a
file from the desktop onto the files and folders in the tree you
get the proper feedback. If you drag the blue canvas onto the tree
you get NO feedback at all.
In fact if you rewrite my entier dragOver event handler and
just have DragManager.showFeedBack(DragManager.NONE) it still
doesn't work for the canvas drag but it does work for desktop