Well...
No matter what I do, it seems the TPanel derived "list container" of mine, do not want to be be dragged - or the Default DragManager do not want to drag my panel. Everything else, but not the one that needs to be dragged.
Either way - I kept my controls functionality, but drag another control - and it works just fine.
So now, when creating my panel, it also creates an invisible TLabel for dragging.
Makes code for accepting drop and dropping a little ... peculiar.
But it overcomes the stubbornness and/or shortcomings of the drag manager.
I would still like an answer to the question, why the DragManager does not want to drag (one of) my Control(s).
What terminates a dragging operaions, besides cancelling and dropping?