Add router.render for waiting on a navigation's data

[pleek91]

Description

Awaiting push only waits for the route to commit, and props and loaders deliberately don't hold up a navigation. That leaves a server with no point at which the page is renderable, and no way to learn that a props getter or loader redirected or rejected. Without waiting it renders markup for a route it has already left; with something awaiting the data, the abandoning navigation rejects that promise and it renders nothing at all.

router.render resolves once there is nothing left to render, following any navigation the data caused, and reports what a server should respond with. start no longer returns anything.

await router.start()

const response = await router.render()

if (response.status >= 300) {
  return respond(response)
}

const html = await renderToString(app)

The response is derived rather than recorded during navigation because the ordinary 404 sets no rejection at all — it commits the NotFound route — so reading the rejection alone would report 200 for most of them.

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