Bug 339573 Splitting the shared area with a part doesn't remove stale placeholders
The current code only looks for the addition of an MPlaceholder in the shared area as the hint to remove locals. When you split the area the element that gets added is the new MPartSashContainer...
bundles/org.eclipse.e4.ui.workbench/src/org/eclipse/e4/ui/internal/workbench/ModelServiceImpl.java
import org.eclipse.e4.ui.model.application.ui.basic.MBasicFactory;
import org.eclipse.e4.ui.model.application.ui.basic.MPart;
import org.eclipse.e4.ui.model.application.ui.basic.MPartSashContainer;
import org.eclipse.e4.ui.model.application.ui.basic.MPartSashContainerElement;
import org.eclipse.e4.ui.model.application.ui.basic.MPartStack;
import org.eclipse.e4.ui.model.application.ui.basic.MTrimBar;
import org.eclipse.e4.ui.model.application.ui.basic.MTrimmedWindow;
import org.eclipse.e4.ui.model.application.ui.basic.MWindow;
import org.eclipse.e4.ui.model.application.ui.basic.MWindowElement;
import org.eclipse.e4.ui.model.application.ui.basic.impl.BasicFactoryImpl;
import org.eclipse.e4.ui.model.application.ui.menu.MToolControl;
import org.eclipse.e4.ui.model.internal.ModelUtils;
import org.eclipse.e4.ui.workbench.modeling.EModelService;
import org.eclipse.emf.ecore.EObject;
/**
 *
 */
public class ModelServiceImpl implements EModelService {
	/**
	 * Determine if the element passes the matching test for all non-null parameters.
	 * 
	 * @param element
	 *            The element to test
	 * @param id
	 *            The Id
	 * @param clazz
	 *            The class that element must be an instance of
	 * @param tagsToMatch
	 *            The tags to check, <b>all</b> the specified rags must be in the element's tags
	 * @return <code>true</code> iff all the tests pass