During a resource serving with cacheability FULL a portlet cannot access the window state and portlet mode (neither the render parameters).
The specification says is vague and say: "Thus the portlet should not access the portlet mode, window state, or render parameters in the serveResource call."
The current Javadoc does not specify anything particular , for example PortletRequest#getPortletMode() say "Returns the current portlet mode of the portlet" .
I think we lack of clarity as we don't specify what PortletRequest#getPortletMode() and PortletRequest#getWindowState() should return in this case.
I believe it should return null and we should say it clearly in the Javadoc, for example we could redeclare those methods in the ResourceRequest interface and add to the javadoc that in case of FULL cacheability then null is returned.