React 16+ changes

React 16.0 relaxed `render()` return types to anything renderable (which `prop-types` and `@types/react` call a "node"), so you should return the observer output as-is, and not wrap in `` (which can break layouts). See https://reactjs.org/blog/2017/09/26/react-v16.0.html

React 16.3 has deprecated `componentWill*` methods (except for `componentWillUnmount`), and will warn if they are used. See https://reactjs.org/blog/2018/03/29/react-v-16-3.html. In my version of this, I simply used the `Did` versions instead, which seems to be mostly fine, but in the new async rendering mode, these might run some time after render, which could be a problem for synchronous observable outputs. Further, `WillUpdate` may run multiple times before an update, though it's not actually described why. The `create-subscription` package the react blog [mentions here](https://reactjs.org/blog/2018/03/27/update-on-async-rendering.html#adding-event-listeners-or-subscriptions) might be a relevant reference (perhaps this library would work as a wrapper on it?)

Start by reading the linked React 16.0 and 16.3 announcements, then inspect the component implementation and its observer-output and lifecycle handling. The issue names no files or tests; done would require a decided approach for renderable outputs and deprecated lifecycle methods, including behavior under asynchronous rendering.

Written by the indexing model from the issue text.