Bug 326177 EPartService's getActivePart() does not appear to work properly for an application with multiple windows
Based on the code in PartServiceCreationFunction, the EPartService implementation being returned when an application's context is queried seems to be a window-level EPartService implementation.
tests/org.eclipse.e4.ui.tests/src/org/eclipse/e4/ui/tests/application/EPartServiceTest.java
application.getChildren().add(window);
		application.setSelectedElement(window);
		MPart partA = BasicFactoryImpl.eINSTANCE.createPart();
		MPart partB = BasicFactoryImpl.eINSTANCE.createPart();
		window.getChildren().add(partA);
		window.getChildren().add(partB);
		window.setSelectedElement(partA);
		
		initialize(applicationContext, application);
		
		getEngine().createGui(window);
		EPartService partService = (EPartService) window.getContext().get(
				EPartService.class.getName());
		partService.activate(partA);
		Object o = applicationContext.get(IServiceConstants.ACTIVE_PART);
		assertEquals(partA, o);
		partService.activate(partB);
		o = applicationContext.get(IServiceConstants.ACTIVE_PART);
		assertEquals(partB, o);
	}
	private void testShowPart_Bug307747(PartState partState) {
		MApplication application = ApplicationFactoryImpl.eINSTANCE
				.createApplication();
		MPartDescriptor partDescriptor = org.eclipse.e4.ui.model.application.descriptor.basic.impl.BasicFactoryImpl.eINSTANCE
				.createPartDescriptor();
		partDescriptor.setElementId("partId");
		partDescriptor.setCategory("category");
		application.getDescriptors().add(partDescriptor);
		final MWindow window = BasicFactoryImpl.eINSTANCE.createWindow();
		application.getChildren().add(window);