The current implementation of SwingDropTarget in TransferHandler registers getDropTargetListener on the parent and keeps all other listeners to itself doing multicast by overriding dragEnter, dragOver, etc.

The way that multicast is implemented is to call super method first and then
notify all the listeners.

There are a couple of glitches there:
1. parent method might notify listener and then do some actions. The way it is
implemented for SwingDropTarget it will notify listener, do action, notify all
the listeners for SwingDropTarget

2. parent method do not notify listeners if not active. SwingDropTarget notify
always.

My suggestion is to implement multicast DropTargetListener and register it with
the super class. All the other listeners plug into that DropTargetListener. This
way the listeners will be invoked the same way as before and at the same conditions.

Say the name for that multicast listener is MultiCastDropTargetListener.
SwingDropTarget.addDropTargetListener, removeDropTargetListener can check
instanceof MultiCastDropTargetListener and call super method if needed.

In this case the constructor for SwingDropTarget could be.
    SwingDropTarget(Component c) {
    super(c, COPY_OR_MOVE | LINK, new MultiCastDropTargetListener());
    addDropTargetListener(getDropTargetLIstener());