Bug 263845 [CommonNavigator] Drag/drop folder from non-Java Proj to Java Proj does copy instead of move sometimes
bundles/org.eclipse.jface/src/org/eclipse/jface/viewers/ViewerDropAdapter.java
private void doDropValidation(DropTargetEvent event) {
    	
    	if (event.detail != DND.DROP_NONE)
    		lastValidOperation = event.detail;
    	
    	currentOperation = lastValidOperation;
    	
        currentEvent = event;
        if (!validateDrop(currentTarget, currentOperation, event.currentDataType)) {
            currentOperation = DND.DROP_NONE;
        }
        event.detail = currentOperation;
        currentEvent = null;
    }
    /* (non-Javadoc)
     * Method declared on DropTargetAdapter.
     * The drag has entered this widget's region.  See
     * if the drop should be allowed.
     */
    public void dragEnter(DropTargetEvent event) {
        currentTarget = determineTarget(event);
        doDropValidation(event);
    }
    /* (non-Javadoc)
     * Method declared on DropTargetAdapter.
     * The drop operation has changed, see if the action
     * should still be enabled.
     */
    public void dragOperationChanged(DropTargetEvent event) {
        currentTarget = determineTarget(event);