DragOverHandler & not accepting drop

According to the HTML5 drag & drop spec, element having dragOver listener and calling preventDefault on it's parameter event tells the browser that this element is a valid drop target.

So, like this:

    function onDragOver(event) {
        event.preventDefault();
    }

If the element does not want to accept the drop, preventDefault is not called.

Now I tried to do the same with GWT's DragOverHandler.

        DragOverHandler dragOverHandler = new DragOverHandler() {
           
            @Override
            public void onDragOver(DragOverEvent event) {
                event.preventDefault();
            }
        };
       
        flowPanel.addBitlessDomHandler(dragOverHandler, DragOverEvent.getType());

Now the problem is that that panel always indicates itself as a drop target, regardless whether I call preventDefault or not. Only by not adding the handler does the panel refuse drop operations. So for some reason just by simply adding a DragOverHandler to a GWT widget makes it accept drag operations. Thus making it impossible for me indicate for user what can be dragged and what not.

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