Bug 326300 EPartService's activate(MPart) method may not work if you activate the window's active part
tests/org.eclipse.e4.ui.tests/src/org/eclipse/e4/ui/tests/application/EPartServiceTest.java
assertEquals(2, perspective.getChildren().size());
		assertEquals(placeholderA, perspective.getChildren().get(0));
		assertEquals(placeholderB, perspective.getChildren().get(1));
	}
	public void testShowPart_Bug321755() {
		MApplication application = ApplicationFactoryImpl.eINSTANCE
				.createApplication();
		MPartDescriptor partDescriptor = org.eclipse.e4.ui.model.application.descriptor.basic.impl.BasicFactoryImpl.eINSTANCE
				.createPartDescriptor();
		partDescriptor.setElementId("partId");
		partDescriptor.setAllowMultiple(true);
		application.getDescriptors().add(partDescriptor);
		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 part = BasicFactoryImpl.eINSTANCE.createPart();
		perspective.getChildren().add(part);
		initialize(applicationContext, application);
		getEngine().createGui(window);
		EPartService partService = window.getContext().get(EPartService.class);
		MPlaceholder placeholder = partService.createSharedPart("partId", true);
		MPart sharedPart = (MPart) placeholder.getRef();