Bug 326087 StackRenderer arbitrarily changes selected element when MPartStack is being unrendered
1. Open two files. 2. Activate the editor in the back. 3. Restart 4. The file in the wrong is active now. :O
tests/org.eclipse.e4.ui.tests/src/org/eclipse/e4/ui/tests/workbench/PartRenderingEngineTests.java
EPartService service = (EPartService) window.getContext().get(
				EPartService.class.getName());
		service.showPart(partB.getElementId(), PartState.ACTIVATE);
		assertEquals("Showing a part should alter the tab folder's selection",
				1, tabFolder.getSelectionIndex());
	}
	public void testPartStack_SetActiveChild3Bug299379() throws Exception {
		MApplication application = ApplicationFactoryImpl.eINSTANCE
				.createApplication();
		application.setContext(appContext);
		appContext.set(MApplication.class.getName(), application);
		MPartDescriptor descriptor = org.eclipse.e4.ui.model.application.descriptor.basic.impl.BasicFactoryImpl.eINSTANCE
				.createPartDescriptor();
		descriptor
				.setContributionURI("platform:/plugin/org.eclipse.e4.ui.tests/org.eclipse.e4.ui.tests.workbench.SampleView");
		descriptor.setElementId("part");
		descriptor.setCategory("aStack");
		application.getDescriptors().add(descriptor);
		MWindow window = BasicFactoryImpl.eINSTANCE.createWindow();
		application.getChildren().add(window);
		MPartStack stack = BasicFactoryImpl.eINSTANCE.createPartStack();
		stack.getTags().add("aStack");
		window.getChildren().add(stack);
		wb = new E4Workbench(application, appContext);
		wb.createAndRunUI(window);
		CTabFolder tabFolder = (CTabFolder) stack.getWidget();
		assertEquals(0, tabFolder.getItemCount());
		EPartService service = (EPartService) window.getContext().get(
				EPartService.class.getName());
		MPart shownPart = service.showPart("part", PartState.ACTIVATE);