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
* 
	 * @see DND#DROP_COPY
	 * @see DND#DROP_MOVE
	 * @see DND#DROP_LINK
	 * @see DND#DROP_NONE
	 * 
	 * @since 3.5
	 */
	protected void overrideOperation(int operation) {
		overrideOperation = operation;
	}
	
    /* (non-Javadoc)
     * Method declared on DropTargetAdapter.
     * The mouse has moved over the drop target.  If the
     * target item has changed, notify the action and check
     * that it is still enabled.
     */
    private void setFeedback(DropTargetEvent event, int location) {
        if (feedbackEnabled) {
            switch (location) {
            case LOCATION_BEFORE:
                event.feedback = DND.FEEDBACK_INSERT_BEFORE;
                break;
            case LOCATION_AFTER:
                event.feedback = DND.FEEDBACK_INSERT_AFTER;
                break;
            case LOCATION_ON:
            default:
                event.feedback = DND.FEEDBACK_SELECT;