Portal root will be at the top of the document body if run during the initial render

[rgossiaux]

This is the exact opposite of what it's supposed to do! But if you have a static <Dialog> you can run into this.

I think it's good enough to just do a tick when initially creating the portal root, and repositioning it at the end of that tick if necessary. That should catch the most glaring cases.

We still won't guarantee that nothing else comes along and adds itself after the portal root, but that's probably desirable-- we don't want to get into a positioning war with some other element.