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
newWindow.setHeight(height);
		element.getParent().getChildren().remove(element);
		MWindowElement uiRoot = wrapElementForWindow(element);
		newWindow.getChildren().add(uiRoot);
		if (curParent instanceof MPerspective) {
			MPerspective persp = (MPerspective) curParent;
			persp.getWindows().add(newWindow);
		} else if (curParent instanceof MWindow) {
			((MWindow) curParent).getWindows().add(newWindow);
		}
	}
	/**
	 * @param element
	 * @return
	 */
	private MWindowElement wrapElementForWindow(MPartSashContainerElement element) {
		if (element instanceof MPlaceholder) {
			MUIElement ref = ((MPlaceholder) element).getRef();
			if (ref instanceof MPart) {
				MPartStack newPS = MBasicFactory.INSTANCE.createPartStack();
				newPS.getChildren().add((MPlaceholder) element);
				return newPS;
			}
		} else if (element instanceof MPart) {
			MPartStack newPS = MBasicFactory.INSTANCE.createPartStack();
			newPS.getChildren().add((MPart) element);
			return newPS;
		} else if (element instanceof MWindowElement) {
			return (MWindowElement) element;
		}