When should the URL change when using transitionWhile? #232

With MPA:

1. Begin fetching
2. Time passes
3. Fetch complete
4. Switch to new page - URL changes here

With transitionWhile:

1. Begin fetching
2. transitionWhile called - URL changes here
3. Time passes
4. Fetch complete
5. Alter DOM

This means old-page content is being shown between steps 2-5, but with the new URL. This presents issues for relative URLs (including links, images, fetch()), which won't behave as expected.

It feels like URL changing should be deferred until the promise passed to transitionWhile resolves, perhaps with an optional commitURLChange() method to perform the swap sooner.