Bug: Resuming a prerender that was aborted while a retried task was rendering drops part of the boundary #37709

React version: main (d083ec1), 19.3.0-experimental-cbb046ab-20260731

Steps To Reproduce

1. Prerender a boundary whose content suspends once, so it gets retried as its own task, and then aborts the prerender synchronously while rendering a later child.
2. Resume the postponed state.

Link to code example:

let resolve;
const promise = new Promise(r => (resolve = r));
let prerendering = true;
const controller = new AbortController();

function Abort() {
  if (prerendering) {
    controller.abort();
  }
  return 'World';
}

function Content() {
  React.use(promise);
  return (
    <section>
      <span>Hello</span>
      <Abort />
    </section>
  );
}

function App() {
  return (
    <div>
      <Suspense fallback="Loading...">
        <Content />
      </Suspense>
    </div>
  );
}

const pending = prerender(<App />, {signal: controller.signal});
resolve();
const {postponed} = await pending;

prerendering = false;
const stream = await resume(<App />, JSON.parse(JSON.stringify(postponed)));

The current behavior

The resumed boundary contains only the child that was rendering when the prerender was aborted: <div>World</div>.

retryRenderTask rewinds the segment to where the attempt started, but task.keyPath and task.childIndex still point at <Abort /> when the task is finished through abortUnwoundTask. trackPostpone records the hole as slot 1 of <section>, and the resume renders <Abort /> alone into the boundary's segment.

We run into this in Next.js 16 with cacheComponents: reading Date.now() in a Client Component aborts the prerender synchronously, the page boundary resumes empty and hydration fails with #418.

The expected behavior

The hole covers everything the aborted attempt discarded, so the resume renders the whole content: <div><section><span>Hello</span>World</section></div>.