fix(svelte): filter stale indices in Virtualizer to prevent undefined data access

[jveres]

fix #851

Summary

• Add data.length dependency to indexes derived to ensure recalculation when data changes
• Filter indices to valid range (index >= 0 && index < len) to prevent stale indices

Problem

During reactive updates (e.g., delete operations), the indexes derived could contain stale indices that exceed the current data.length, causing "Cannot
read properties of undefined" errors when accessing data[index] in the #each block.

Solution

By accessing data.length in the indexes derived, we create a reactive dependency that ensures indexes recalculates atomically with data changes. The
filtering ensures only valid indices are included.

Test plan

• Create structural changes (e.g. delete) in items of a virtualized list with Svelte 5
• Verify no runtime exceptions occur

[inokawa]

Hi, could you provide a reproducible code for the problem?

[jveres]

Here's a minimal reproduction you can paste into the Svelte playground:

<script>
    import { VList } from "virtua/svelte";

    let items = $state(
      Array.from({ length: 100 }, (_, i) => ({ id: i, text: `Item ${i}` }))
    );

    function deleteItem(id) {
      items = items.filter((item) => item.id !== id);
    }
</script>

<!--
  Steps to reproduce:
  1. Scroll down to the bottom of the list (items 95-99)
  2. Click "Delete" on any visible item
  3. BUG: "Cannot read properties of undefined" error

  The range still contains indices 95-99, but after deletion
  data.length is 99, so data[99] is undefined.
-->

<div style="height: 400px; overflow: auto;">
  <VList data={items} getKey={(item) => item.id}>
    {#snippet children(item, index)}
      <div style="height: 50px; padding: 8px; border-bottom: 1px solid #ccc; display: flex; justify-content: space-between; align-items: center;">
        <span>{item.text}</span>
        <button onclick={() => deleteItem(item.id)}>Delete</button>
      </div>
    {/snippet}
  </VList>
</div>

Steps to trigger the bug:

1. Scroll to the very bottom of the list (so items 95-99 are visible)
2. Click "Delete" on any item
3. You'll see "Cannot read properties of undefined" because indexes contains stale indices that exceed the new data.length

[inokawa]

Shipped in 0.48.6. Thanks!