Bug 307747 EPartService doesn't render new parts
1. Have a part in a stack, it is the selected part. 2. Show another part in that stack using PartState.CREATE or PartState.VISIBLE. 3. The part should've been rendered. It is not.
bundles/org.eclipse.e4.ui.workbench/src/org/eclipse/e4/workbench/ui/internal/PartServiceImpl.java
if (providedPart != localPart && !descriptor.isAllowMultiple()) {
				return localPart;
			}
			String category = descriptor.getCategory();
			if (category == null) {
				addToLastContainer(null, providedPart);
			} else {
				List<Object> elements = modelService.findElements(rootContainer, null, null,
						Collections.singletonList(category));
				if (elements.isEmpty()) {
					addToLastContainer(category, providedPart);
				} else {
					Object element = elements.get(0);
					if (element instanceof MElementContainer<?>) {
						((MElementContainer<MPart>) element).getChildren().add(providedPart);
					} else {
						addToLastContainer(category, providedPart);
					}
				}
			}
		}
		return providedPart;
	}
	private void addToLastContainer(String category, MPart part) {
		MElementContainer<?> lastContainer = getLastContainer();
		((List) lastContainer.getChildren()).add(part);
		if (category != null) {
			lastContainer.getTags().add(category);
		}
	}