TypedArray updates not working in Runes Mode

[hykilpikonna]

Describe the bug

TLDR:

Screen.Recording.2024-12-18.125121.x264.25.mp4

In legacy mode, I could update one element of a TypedArray and the corresponding update would be O(1):

<script lang="ts">
  let n = 10 ** 7
  
  let count: Int8Array = new Int8Array(n)
  const increment = () => {
    count[0] += 1
  }
</script>

<button onclick={increment}>
  count is {count[0]}
</button>

In Runes mode, I cannot do that. The following code does not work. The array is updated but the reactivity is lost, no DOM elements are updated:

<script lang="ts">
  let n = 10 ** 7

  let count: Int8Array = $state(new Int8Array(n))
  const increment = () => {
    count[0]++
  }
</script>

<button onclick={increment}>
  count is {count[0]}
</button>

If I however copy the array on each call, DOM elements are updated but it takes significantly more time (an O(N) operation for a O(1) change)

<script lang="ts">
  let n = 10 ** 7

  let count: Int8Array = $state(new Int8Array(n))
  const increment = () => {
    // Copy array
    const tmp = Int8Array.from(count)
    tmp[0]++
    count = tmp
  }
</script>

<button onclick={increment}>
  count is {count[0]}
</button>

Reproduction

https://svelte.dev/playground/9470398dceac4efebf694c307a04b452?version=5.14.4

Logs

No response

System Info

System:
    OS: Windows 11 10.0.22631
    CPU: (16) x64 AMD Ryzen 7 7840HS with Radeon 780M Graphics
    Memory: 3.75 GB / 27.69 GB
  Binaries:
    Node: 22.5.1 - C:\Program Files\nodejs\node.EXE
    Yarn: 1.22.22 - ~\AppData\Roaming\npm\yarn.CMD
    npm: 10.8.2 - C:\Program Files\nodejs\npm.CMD
    bun: 1.1.38 - ~\.bun\bin\bun.EXE
  Browsers:
    Edge: Chromium (127.0.2651.74)
    Internet Explorer: 11.0.22621.3527

Severity

blocking an upgrade

[Conduitry]

See #14639 and other linked issues for a draft/proposal of a feature to support this. As it stands, this would fall under the same category as other non-POJOs not having automatic reactivity.