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.swt/src/org/eclipse/e4/ui/internal/workbench/swt/PartRenderingEngine.java
SafeRunner.run(new ISafeRunnable() {
			public void handleException(Throwable e) {
				if (e instanceof Error) {
					
					throw (Error) e;
				} else {
					
					if (logger != null) {
						String message = "Exception occurred while unrendering: {0}"; 
						logger.error(e, NLS.bind(message, element));
					}
				}
			}
			public void run() throws Exception {
				safeRemoveGui(element);
			}
		});
	}
	private void safeRemoveGui(MUIElement element) {
		if (removeRoot == null)
			removeRoot = element;
		renderedElements.remove(element);
		
		
		MUIElement parent = element.getParent();
		AbstractPartRenderer parentRenderer = parent != null ? getRendererFor(parent)
				: null;
		if (parentRenderer != null) {