Bug 151959 inappropriate screen operation
steps to reproduce: - create a report - add a grid item (3x3) - switch to Outline view - expand the grid item to see its rows and cells - select the grid item, and drag it into any of its own cell expected behavior: - is it an invalid screen operation? if it is, it should be banned actual behavior: - the grid item was disappeared surprisingly
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/core/DesignElement.java
* Inverse of the extends relationship: parent-->derived. Very few elements
	 * are extended, so we create this list only when needed.
	 */
	protected ArrayList derived = null;
	/**
	 * The unique ID assigned to this element when it is added to the design.
	 * Allows web applications to refer to the element by ID.
	 */
	protected long id = NO_ID;
	/**
	 * Cached element definition. Cached for speed since the definition cannot
	 * change.
	 */
	protected IElementDefn cachedDefn = null;
	/**
	 * Indicates whether the element is valid. The initial value is true.
	 */
	protected boolean isValid = true;
	/**
	 * API handle for this element.
	 */
	protected DesignElementHandle handle = null;
	/**
	 * The validation error list, each of which is the instance of
	 * <code>SemanticException</code>.
	 */
	protected List errors;
	/**
	 * The set of slots for the design element. If the design element has no
	 */