React rendering sometimes shows index page

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e.g. if you render a component then move the cell elsewhere in the notebook

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also if you restart the kernel

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showing the index page was somehow fixed along the way, the request to Vite now 404s. (When you move a cell it gets a new cell ID, but the output isn't updated, so it refers to the old ID.)

It still looks strange; the output is blank, but the container doesn't return to its original size (this is because the resize observer and its handler aren't loaded).

Maybe it's possible to keep the old cell ID on a move. And maybe we could render a dummy output instead of 404ing when the cell doesn't exist, so we could fix the container size.