[🐞] <Pending> child reading a signal from its owner breaks when the owner re-renders (consumerBackRef[_EFFECT_BACK_REF].set is not a function) #9166

Which component is affected?

Qwik Runtime

Describe the bug

Which component is affected?

Qwik Runtime

Describe the bug

A component inside <Pending> reads a signal passed in as a prop by the component that renders the <Pending>. When that owner re-renders on the client, the update breaks:

• Dev: consumerBackRef[_EFFECT_BACK_REF].set is not a function is thrown and the owner's DOM change never appears.
• Production: no error, but the <Pending> fallback becomes visible and the child is rendered a second time next to the original.

No async work or out-of-order streaming is needed: the content renders in order and nothing ever suspends.

These don't trigger it:

• <Pending> with only static content.
• Reading the signal inline inside <Pending> (<p>{text.value}</p>).
• A child that reads its own signal instead of one passed from the owner.
• The same tree without <Pending>.

Reproduction

https://stackblitz.com/edit/github-u5aqdquk-66auz4fv?file=src%2Froutes%2Findex.tsx

import { component$, Pending, useSignal, type Signal } from '@qwik.dev/core';

const Reader = component$<{ value: Signal<string> }>(({ value }) => <p>{value.value}</p>);

// The owner re-renders structurally while a child inside <Pending> reads its signal.
export default component$(() => {
  const on = useSignal(false);
  const text = useSignal('hello');
  return (
    <>
      <button onClick$={() => (on.value = !on.value)}>toggle</button>
      {on.value && <b>on</b>}
      <Pending fallback$={() => <p>loading…</p>}>
        <Reader value={text} />
      </Pending>
    </>
  );
});

Steps to reproduce

1. Open the reproduction (experimental: ['pendingBoundary'] is enabled in qwikVite()).
2. Click toggle.

Expected: <b>on</b> appears; nothing else changes.

Actual (dev): the console shows QWIK ERROR consumerBackRef[_EFFECT_BACK_REF].set is not a function and <b>on</b> never appears.

Actual (production build): no error, but after the click the page shows <b>on</b>, the visible fallback <p>loading…</p>, and a second <p>hello</p> beside the original.

System Info

@qwik.dev/core   2.0.0-rc.0 (main@5c51580)
@qwik.dev/router 2.0.0-rc.0 (main@5c51580)
vite             8.2.2
node             22.x
macOS

Additional Information

It behaves the same with #9147, #9163 and #9165 applied, so it isn't related to those out-of-order streaming fixes.

Reproduction

https://stackblitz.com/edit/github-u5aqdquk-66auz4fv?file=src%2Froutes%2Findex.tsx

Steps to reproduce

No response

System Info

System:
    OS: Linux 5.0 undefined
    CPU: (8) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 0 Bytes / 0 Bytes
    Shell: 1.0 - /bin/jsh
  Binaries:
    Node: 22.22.3 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 10.8.2 - /usr/local/bin/npm
    pnpm: 8.15.6 - /usr/local/bin/pnpm
  npmPackages:
    @qwik.dev/core: ^2.0.0-rc.0 => 2.0.0-rc.0 
    @qwik.dev/router: ^2.0.0-rc.0 => 2.0.0-rc.0 
    typescript: 5.9.3 => 5.9.3 
    vite: 8.1.5 => 8.1.5

Additional Information

No response