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#detach(org.eclipse.e4.ui.model.application
	 * .MPartSashContainerElement)
	 */
	public void detach(MPartSashContainerElement element, int x, int y, int width, int height) {
		
		MUIElement curParent = element.getParent();
		MUIElement current = element;
		MWindow window = getTopLevelWindowFor(element);
		while (!(curParent instanceof MPerspective) && !(curParent instanceof MWindow)) {
			if (curParent == null) {
				
				current = findPlaceholderFor(window, current);
				if (current == null) {
					return; 
				}
				curParent = current.getParent();
				if (curParent == null) {
					return; 
				}
			}
			current = curParent;
			curParent = current.getParent();
		}
		MTrimmedWindow newWindow = BasicFactoryImpl.INSTANCE.createTrimmedWindow();
		
		newWindow.setX(x);
		newWindow.setY(y);
		newWindow.setWidth(width);