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
* User-defined elements can extend other elements, but style element is not
 * allowed to extend each other. The following is the terminology used for this
 * system.
 * <ul>
 * <li>If element E extends element D, then D is said to be the
 * <em>parent </em> element, and element E is the <em>derived </em> element.
 * </li>
 * <li>We can also use the term <em>base </em> as a synonym for parent. That
 * is, D is the base element above.</li>
 * <li>Element D may in turn extend another element C.</li>
 * <li>We say that C and D are <em>ancestor </em> elements of E, and that D
 * and E are <em>descendents </em> of C.</li>
 * <li>In this simple hierarchy, element E is the <em>most derived </em>
 * element, while element C is the <em>most base </em> element.</li>
 * </ul>
 * <p>
 * Inheritance applies to the following:
 * <p>
 * <dl>
 * <dt><strong>Property values </strong></dt>
 * <dd>An element inherits any property values defined by any of its ancestors.
 * If two ancestors define values for the same property, then the inherited
 * value is that defined by the most derived ancestor. This rule is similar to
 * the method overriding rules in Java.</dd>
 * 
 * <dt><strong>Named style </strong></dt>
 * <dd>If an ancestor element explicitly names a style, then the derived
 * element will also use that style (unless the derived element specifies a
 * different style.)</dd>
 *