Bug 327964 EPartService's hidePart may fail when trying to hide a shared part that's in a shared container
The placeholder retrieval code in the EPartService implementation doesn't go far down deep enough and actually stops at the shared container level. This causes the placeholder of the shared part to not be found.
tests/org.eclipse.e4.ui.tests/src/org/eclipse/e4/ui/tests/application/EPartServiceTest.java
application.setSelectedElement(window);
		MPartStack stackA = BasicFactoryImpl.eINSTANCE.createPartStack();
		window.getChildren().add(stackA);
		window.setSelectedElement(stackA);
		MPartStack stackB = BasicFactoryImpl.eINSTANCE.createPartStack();
		window.getChildren().add(stackB);
		MPart partA = BasicFactoryImpl.eINSTANCE.createPart();
		stackA.getChildren().add(partA);
		stackA.setSelectedElement(partA);
		MPart partB1 = BasicFactoryImpl.eINSTANCE.createPart();
		stackB.getChildren().add(partB1);
		stackB.setSelectedElement(partB1);
		MPart partB2 = BasicFactoryImpl.eINSTANCE.createPart();
		stackB.getChildren().add(partB2);
		MPart partB3 = BasicFactoryImpl.eINSTANCE.createPart();
		stackB.getChildren().add(partB3);
		initialize(applicationContext, application);
		getEngine().createGui(window);
		EPartService partService = window.getContext().get(EPartService.class);
		partService.activate(partB2);
		partService.activate(partB3);
		partService.activate(partA);
		partService.hidePart(partB3);
		assertEquals(partB2, stackB.getSelectedElement());
	}
	public void testHidePart_ActivationHistory02() {
		MApplication application = ApplicationFactoryImpl.eINSTANCE
				.createApplication();
		MWindow window = BasicFactoryImpl.eINSTANCE.createWindow();
		application.getChildren().add(window);