Svelte getKey is called with undefined when data length decreases

[Lechros]

Describe the bug
When the data array passed to <Virtualizer> length decreases, getKey can be invoked with undefined for some indices.

Uncaught TypeError: Cannot read properties of undefined (reading 'id')

	in <unknown>
	in Virtualizer.svelte
	in +page.svelte
	in layout.svelte
	in root.svelte
	in undefined

    at getKey (+page.svelte:10:53)

To Reproduce
Minimal Svelte 5 example:

<script lang="ts">
  import { Virtualizer } from "virtua/svelte";

  let data = $state(Array.from({ length: 5 }, (_, i) => ({ id: i })));
</script>

<button type="button" onclick={() => data.pop()}>pop()</button>

<div>
  <Virtualizer {data} getKey={(item, index) => item.id}>
    {#snippet children(item, index)}
      {item}
    {/snippet}
  </Virtualizer>
</div>

Expected behavior
getKey should only be called with a valid data[index] for the current data.

Platform:

• OS: Windows
• Browser: Chrome
• Version of this package: 0.48.5
• Version of framework: svelte 5.51.2

Additional context
I think it was fine in virtua 0.41.5.

AI analysis:

• The mismatch occurs in Virtualizer.svelte at the {#each indexes as index (getKey(data[index]!, index))} line: indexes comes from $derived(store.$getRange(...)), while the store’s length is updated only inside $effect.pre. So when the parent updates data to a shorter array in the same tick, the component runs with new data but old indexes, and the each block calls getKey(data[index], index) for indices that are out of bounds for the new data.
• Same underlying issue can appear with React/Vue/Solid if the framework’s effect/update order causes data to update before the virtual store’s length is applied in the same frame.

[inokawa]

Not fully investigated but #847 may fix this issue.

[inokawa]

Shipped in 0.48.6.