[2.0 / next] A memo that read latest(x) last run holds a Loading on switch even when its new run no longer reads x #4661

On next @ 6f33602.

A memo outside a <Loading on={f()}> reads f() and, in the branch taken before the switch, latest(() => list()). When f changes, the new run takes the other branch and does not read list at all. The switch is still held silently until list's new answer lands, so the Loading on fallback never shows and the old content stays on screen. The same memo with untrack in place of latest doesn't hold. A memo that always reads latest(() => list()) doesn't hold either. It holds whether the memo is read by an effect or by a view outside the boundary.

const [f, setF] = createSignal("open");
const list = createMemo(() => ask(`list:${f()}`));            // a promise per key
const count = createMemo(() => f() === "open" ? latest(() => list().length) : -1);
createEffect(() => count(), v => { seen = v; });
return <Loading fallback={<i>first</i>}>
  <Loading on={f()} fallback={<i>loading {f()}</i>}><b>{list().join(",")}</b></Loading>
</Loading>;

Steps: land list:open = [1,2,3], then setF("closed") and flush.

When list:closed lands, every variant shows 9 and the count -1.

How we found it: ReactiveScript's checker suggests "read latest x here" for a reader outside a loading on(k) region. An app counting open issues, filter == "open" ? (latest shown).length : -1, still froze on the filter switch. Three agents hit it independently and each spent several turns working around it.