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Preview: https://svelte-dev-git-preview-kit-14129-svelte.vercel.app/

I don't think we can do this. Today, if you try and do something like this...

<script>
  import { page } from '$app/state';
  import { pushState } from '$app/navigation';
</script>

<section>
  <button
    onclick={() => {
      let object = $state({ count: 0 });
      pushState('/', { object });
    }}
  >
    push state
  </button>

  {#if page.state?.object}
    <p>{page.state.object.count}</p>

    <button onclick={() => (page.state.object.count += 1)}>
      increment page.state.object.count
    </button>
  {/if}
</section>

...SvelteKit will yell at you, because you can't serialize a proxy. With this PR, if you click the first button, page.state.object becomes a reactive proxy. You can increment page.state.object.count by clicking the button. But if you navigate back, and then forward, object.count will be a non-reactive deserialized object. And so on. It's basically making a promise it can't keep. I think the current approach is probably better, even if (or maybe because) it's more restrictive

(Technically it's the browser that yells at me if I give it a proxy, devalue will happily serialize it)

Would it be acceptable to $state.snapshot the value given to pushState? For your specific example, the behavior would then be consistent before and after a history state pop (I guess)

In dev we can also add a warning that reactivity will be lost for history states if that makes the behavior less surprising

@Rich-Harris I fixed it the stupid way™, what do you think?

Well it's not that far from the fix in #14268, maybe it's not that dumb

Quick (and long overdue — apologies) update: our latest thinking on this is that we should use IndexedDB instead of sessionStorage, since it handles these cases without needing any serialization at all (and gives us a much larger quota in the process). Groundwork in #16346