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
MPartStack partStack = (MPartStack) window.getChildren().get(0);
		MPart partFront = (MPart) partStack.getChildren().get(0);
		MPart partBack = (MPart) partStack.getChildren().get(1);
		partStack.setSelectedElement(partFront);
		getEngine().createGui(window);
		EPartService partService = (EPartService) window.getContext().get(
				EPartService.class.getName());
		assertEquals(partFront, partService.getActivePart());
		PartListener partListener = new PartListener();
		partService.addPartListener(partListener);
		partService.activate(partBack);
		assertEquals(1, partListener.getVisible());
		assertEquals(partBack, partListener.getVisibleParts().get(0));
		assertTrue(partListener.isValid());
		partListener.clear();
		partService.activate(partFront);
		assertEquals(1, partListener.getVisible());
		assertEquals(partFront, partListener.getVisibleParts().get(0));
		assertTrue(partListener.isValid());
	}
	public void testEvent_PartVisible2() {
		MApplication application = ApplicationFactoryImpl.eINSTANCE
				.createApplication();
		MWindow windowA = BasicFactoryImpl.eINSTANCE.createWindow();
		MWindow windowB = BasicFactoryImpl.eINSTANCE.createWindow();
		application.getChildren().add(windowA);
		application.getChildren().add(windowB);
		application.setSelectedElement(windowA);
		MPartStack stack = BasicFactoryImpl.eINSTANCE.createPartStack();
		windowB.getChildren().add(stack);