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
partDescriptor.setCategory("categoryId");
		partDescriptor.setElementId("partId2");
		application.getDescriptors().add(partDescriptor);
		MPartStack stack = BasicFactoryImpl.eINSTANCE.createPartStack();
		stack.getTags().add("categoryId");
		window.getChildren().add(stack);
		initialize(applicationContext, application);
		getEngine().createGui(window);
		EPartService partService = (EPartService) window.getContext().get(
				EPartService.class.getName());
		MPart part = partService.showPart("partId", PartState.ACTIVATE);
		assertEquals(1, stack.getChildren().size());
		assertEquals(part, stack.getChildren().get(0));
		assertEquals(part, stack.getSelectedElement());
		MPart part2 = partService.showPart("partId2", PartState.ACTIVATE);
		assertEquals(2, stack.getChildren().size());
		assertEquals(part, stack.getChildren().get(0));
		assertEquals(part2, stack.getChildren().get(1));
		assertEquals(part2, stack.getSelectedElement());
	}
	public void testShowPart_Id_CREATE() {
		MApplication application = ApplicationFactoryImpl.eINSTANCE
				.createApplication();
		MWindow window = BasicFactoryImpl.eINSTANCE.createWindow();
		application.getChildren().add(window);
		MPartStack partStackA = BasicFactoryImpl.eINSTANCE.createPartStack();
		MPartStack partStackB = BasicFactoryImpl.eINSTANCE.createPartStack();
		window.getChildren().add(partStackA);
		window.getChildren().add(partStackB);
		MPart partA1 = BasicFactoryImpl.eINSTANCE.createPart();