Bug 322356 EPS's isPartVisible(MPart) method does not consider placeholders
SSIA
bundles/org.eclipse.e4.ui.workbench/src/org/eclipse/e4/ui/internal/workbench/PartServiceImpl.java
MPartStack stack = BasicFactoryImpl.eINSTANCE.createPartStack();
				if (providedPart.getCurSharedRef() instanceof MPlaceholder) {
					stack.getChildren().add(providedPart.getCurSharedRef());
				} else {
					stack.getChildren().add(providedPart);
				}
				rootContainer.getChildren().add(stack);
			}
		} else {
			if (providedPart != localPart && !descriptor.isAllowMultiple()) {
				return localPart;
			}
			if (isInContainer(providedPart)) {
				return providedPart;
			}
			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<?>) {
						MPlaceholder placeholder = providedPart.getCurSharedRef();
						if (placeholder == null) {
							((MElementContainer) element).getChildren().add(providedPart);
						} else {