[svelte-query] A result key first read after the query settled stays stale: the component's copy refreshes only on a change of an already-tracked key #12485

Versions: @tanstack/svelte-query 6.3.1, @tanstack/query-core 5.104.1, Svelte 5.

What happens

When notifyOnChangeProps is unset, createBaseQuery wraps the result in observer.trackResult(result) (createBaseQuery.svelte.js:29). QueryObserver.updateResult then notifies only when a key in #trackedProps changes (queryObserver.js:395-403).

The observer does add a key to its tracked set when a template first reads it. But the component's reactive copy is updated only through those notifications. If a query settles while the template has read only some of its keys, the update is skipped and the component keeps its copy of the old result. A key that the template reads later is then read from that stale copy, and nothing updates it again, because the query has already settled.

Minimal case

<script lang="ts">
  import { createQuery } from '@tanstack/svelte-query';

  const wait = (ms: number) => new Promise((resolve) => setTimeout(resolve, ms));
  const a = createQuery(() => ({ queryKey: ['a'], queryFn: () => wait(300).then(() => 'a') }));
  const b = createQuery(() => ({ queryKey: ['b'], queryFn: () => Promise.resolve('b') }));
</script>

{#if a.isError || b.isError}
  <p>error</p>
{:else if a.data === undefined || b.data === undefined}
  <p>loading</p>
{:else}
  <p>{a.data} {b.data}</p>
{/if}

Expected: a b after about 300 ms.

Actual: loading stays forever. b settles first, while the template has read only b.isError, because the || short-circuits before b.data. The change to b.data doesn't notify, so the component's copy of b keeps data: undefined. When a settles, the template reads b.data from that stale copy.

Workaround

Set notifyOnChangeProps: 'all' in the client's default query options, or read every key before branching.

Suggestion

Pass onPropTracked to trackResult and refresh the component's copy when a key is first tracked, or skip tracking in the Svelte adapter, since Svelte already re-renders key by key.