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
* 
	 * @param child
	 *            The old derived element.
	 */
	public void dropDerived( DesignElement child )
	{
		assert derived != null;
		assert child != null;
		assert child.getExtendsElement( ) == this;
		assert derived.contains( child );
		derived.remove( child );
	}
	/**
	 * Sets the parent element. This version should be used to implement a
	 * command to change the parent element. The caller must have validated that
	 * the new parent is valid for this element.
	 * <p>
	 * Part of: Inheritance system.
	 * 
	 * @param base
	 *            The new parent element, or null if the element is to have no
	 *            base element.
	 */
	public void setExtendsElement( DesignElement base )
	{
		DesignElement oldExtends = getExtendsElement( );
		if ( base == oldExtends && base != null )
			return;
		if ( oldExtends != null )
			oldExtends.dropDerived( this );