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
engine.applyStyles((Widget) widget, true);
				}
				public void style(Object widget) {
					engine.applyStyles((Widget) widget, true);
				}
				public CSSStyleDeclaration getStyle(Object widget) {
					Element e = engine.getCSSElementContext(widget)
							.getElement();
					if (e == null) {
						return null;
					}
					return engine.getViewCSS().getComputedStyle(e, null);
				}
				public void setClassnameAndId(Object widget, String classname,
						String id) {
					((Widget) widget).setData(
							"org.eclipse.e4.ui.css.CssClassName", classname); 
					((Widget) widget).setData("org.eclipse.e4.ui.css.id", id); 
					engine.applyStyles((Widget) widget, true);
				}
			});
			URL url;
			InputStream stream = null;
			try {
				url = FileLocator.resolve(new URL(cssURI));
				stream = url.openStream();
				engine.parseStyleSheet(stream);
			} catch (MalformedURLException e) {
				
				e.printStackTrace();