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
* @see org.eclipse.e4.ui.workbench.modeling.EModelService#getContainer(org.eclipse.e4.ui.model.
	 * application.ui.MUIElement)
	 */
	public MUIElement getContainer(MUIElement element) {
		if (element == null)
			return null;
		return (MUIElement) ((EObject) element).eContainer();
	}
	public int getElementLocation(MUIElement element) {
		if (element == null)
			return NOT_IN_UI;
		MUIElement curElement = element;
		while (curElement != null) {
			MUIElement parent = curElement.getParent();
			if (parent instanceof MPerspective) {
				MElementContainer<MUIElement> perspectiveParent = parent.getParent();
				if (perspectiveParent == null)
					return NOT_IN_UI;
				else if (perspectiveParent.getSelectedElement() == parent)
					return IN_ACTIVE_PERSPECTIVE;
				else
					return IN_ANY_PERSPECTIVE;
			} else if (parent instanceof MApplication) {
				return OUTSIDE_PERSPECTIVE;
			} else if (parent instanceof MTrimBar) {
				return IN_TRIM;
			} else if (parent == null) {
				EObject container = ((EObject) curElement).eContainer();
				
				if (container instanceof MWindow) {