Timing of effects

Although useEffect is deferred until after the browser has painted, it's guaranteed to fire before any new renders. React will always flush a previous render's effects before starting a new update.

Is this true in both concurrent and non-concurrent mode. It would appear intuitively that these two call-sites in non-concurrent mode would be deterministic:

ReactDOM.render(<Foo>, container)
ReactDOM.render(<Foo>, container)

That is it would invoke a new render before the effect in the first render has been invoked.

If so would it make more sense to specify it is only in relation to effects i.e previous effects are guaranteed to run before future effects.

As it stands, it looks appears as though the "render" call-site is non-deterministic in non-concurrent.

Start by reviewing the quoted documentation statement about effects being flushed before new renders, then compare its wording with the two ReactDOM.render call sites and both concurrent and non-concurrent behavior. Done means the documentation clearly states the supported guarantee and resolves whether it applies to renders generally or only to effect ordering.