[2.0 / next] latest(m2) reveals a lane's answer while m1 of the same lane is still in flight (F-6) #4408

Summary

Two latest() async readers sit under one async memo. s is written, and t about 12 ms later while the s flush is parked. A render effect showing latest(m2) (where m2 = m0b() + 1) shows c1101 while m1:1122 of the same lane is still in flight. Under F-6 (amended 2026-10-28: the parent waits for the lane's flights) and D-26, the lane's readers show its answer together, so c1101 must not show before v1122.

Reproduction

const [s, setS] = createSignal(1);
const [t, setT] = createSignal(0);
const ask = (n, v) => (log(`${n}:${v}`), sleep(10).then(() => v));
const m0 = createMemo(() => ask("m0", 1 + 10 * latest(s) + s()));
const m0b = createMemo(() => ask("m0b", 100 * (10 * latest(t) + t())));
const m1 = createMemo(() => ask("m1", m0() + m0b()));
const m2 = createMemo(() => m0b() + 1);
createLoadingBoundary(() => {
  createRenderEffect(() => `v${m1()}`, show);
  createRenderEffect(() => `c${latest(m2)}`, show);
  return null;
}, () => "L");
// settle, then:
setS(2); flush(); await sleep(12); flush();
setT(1); flush();
// settle

Pinned as it.fails in packages/signals/tests/latest-two-readers-one-ask-4393.test.ts ("m1's lane pass is not re-derived over a staging that lands with it (beside)") with #4393.

Observed

c1101 shows while m1:1122 is in flight, before v1122.

Expected

v1122 shows no later than c1101, and after c1101 the only v value shown is v1123. Each question is asked once.

Notes

• Fails on next as well. fix(signals): latest() opens no transaction for its verdict; its reader watches the seam (#4365) #4393 fixes the other rows of this family (each question asked once for the pair, split, value and screen readers) but not this one.
• Cause, as far as we traced it: fix(signals): an async memo over a verdict memo asks each question once #4397 makes m1 a verdict reader between its flights (D-25), so m0's lane pass re-derives it into the s lane early. The lane is unshown when m1:22 lands, nothing holds the t transaction, and c1101 shows. Telling "its hold re-runs it" (D-25) apart at that landing needs a per-pass check, so this is a row for the reader-seat carve.