Bug 232777 [Viewers] ViewerDropAdapter holds currentTarget forever
Build ID: 3.3.2 Member currentTarget should be set to null after drop or after cancel, because it holds a domain object, which can't be garbage collected after Viewer.setInput(null).
bundles/org.eclipse.jface/src/org/eclipse/jface/viewers/ViewerDropAdapter.java
/**
     * The viewer to which this drop support has been added.
     */
    private Viewer viewer;
    /**
     * The current operation.
     */
    private int currentOperation = DND.DROP_NONE;
    /**
     * The last valid operation.  We need to remember the last good operation
     * in the case where the current operation temporarily is not valid (drag over
     * someplace you can't drop).  
     */
    private int lastValidOperation;
    
    /**
     * This is used because we allow the operation 
     * to be temporarily overridden (for example a move to a copy) for a drop that
     * happens immediately after the operation is overridden.
     */
    private int overrideOperation = -1;
    
    /**
     * The current DropTargetEvent, used only during validateDrop()
     */
    private DropTargetEvent currentEvent;
    
    /**
     * The data item currently under the mouse.
     */