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
partStack.getChildren().add(placeholderC);
		partStack.setSelectedElement(placeholderC);
		initialize(applicationContext, application);
		getEngine().createGui(window);
		assertNull(partB.getObject());
		EPartService partService = window.getContext().get(EPartService.class);
		partService.activate(partA);
		partService.hidePart(partA);
		assertEquals(partC, partService.getActivePart());
	}
	public void testHidePart_ActivationHistory_Bug327952_01() {
		MApplication application = ApplicationFactoryImpl.eINSTANCE
				.createApplication();
		MWindow window = BasicFactoryImpl.eINSTANCE.createWindow();
		application.getChildren().add(window);
		application.setSelectedElement(window);
		MPerspectiveStack perspectiveStack = AdvancedFactoryImpl.eINSTANCE
				.createPerspectiveStack();
		window.getChildren().add(perspectiveStack);
		window.setSelectedElement(perspectiveStack);
		MPerspective perspective = AdvancedFactoryImpl.eINSTANCE
				.createPerspective();
		perspectiveStack.getChildren().add(perspective);
		perspectiveStack.setSelectedElement(perspective);
		MPart partA = BasicFactoryImpl.eINSTANCE.createPart();
		window.getChildren().add(partA);
		window.setSelectedElement(partA);
		MWindow detachedWindow = BasicFactoryImpl.eINSTANCE.createWindow();
		perspective.getWindows().add(detachedWindow);
		MPart partB = BasicFactoryImpl.eINSTANCE.createPart();