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
* defined in a subclass of this class.
 * <p>
 * However, all content elements have at most one container. And, all elements
 * (except the root) have a container. So, the content-to-container relationship
 * is modelled in this element base class.
 * <p>
 * Containers have one or more <em>slots </em>. See the
 * {@link org.eclipse.birt.report.model.metadata.SlotDefn}class for a detailed
 * description of slots.
 * <p>
 * Members in the containment system include:
 * <p>
 * <ul>
 * <li>The {@link #container}member that caches a pointer to the container
 * element.</li>
 * <li>The {@link #getContainer}method to get the container element.</li>
 * <li>The {@link #setContainer}method to set the container element.</li>
 * <li>The {@link #getSlot}method returns a slot within a container.
 * <li>The {@link ContainerSlot},{@link SingleElementSlot}and
 * {@link MultiElementSlot}classes that represent slots.</li>
 * </ul>
 * 
 * <h3>Change Notification</h3>
 * 
 * Any given element may be "visualized" in multiple places in the user
 * interface. If the user changes the element, all the affected parts of the UI
 * must be updated. The notification system is responsible for these updates.
 * <p>
 * The notification system allows any number of {@link Listener}objects to
 * subscribe to receive change notifications for an element. Notifications are