Bug 327964 EPartService's hidePart may fail when trying to hide a shared part that's in a shared container
The placeholder retrieval code in the EPartService implementation doesn't go far down deep enough and actually stops at the shared container level. This causes the placeholder of the shared part to not be found.
bundles/org.eclipse.e4.ui.workbench/src/org/eclipse/e4/ui/internal/workbench/PartServiceImpl.java
sharedPart = createPart(descriptor);
			if (sharedPart == null) {
				return null;
			}
			sharedWindow.getSharedElements().add(sharedPart);
		}
		return createSharedPart(sharedPart);
	}
	private MPlaceholder createSharedPart(MPart sharedPart) {
		
		MPlaceholder sharedPartRef = AdvancedFactoryImpl.eINSTANCE.createPlaceholder();
		sharedPartRef.setElementId(sharedPart.getElementId());
		sharedPartRef.setRef(sharedPart);
		return sharedPartRef;
	}
	private MPart addPart(MPart providedPart, MPart localPart) {
		
		if (providedPart == localPart && isInContainer(providedPart)) {
			return providedPart;
		}
		MPartDescriptor descriptor = findDescriptor(providedPart.getElementId());
		if (descriptor == null) {
			if (!isInContainer(providedPart)) {
				adjustPlaceholder(providedPart);
				MPartStack stack = BasicFactoryImpl.eINSTANCE.createPartStack();
				MPlaceholder placeholder = providedPart.getCurSharedRef();
				if (placeholder != null) {
					stack.getChildren().add(placeholder);
				} else {
					stack.getChildren().add(providedPart);