latest inside Show stays stale while removal is pending

[GabbeV]

Describe the bug

Two paragraphs read latest(count), one inside Show and one outside. While an action is pending, Outside becomes 1 but the still-visible Inside remains 0.

Your Example Website or App

https://s.olid.uk/id/r5C3uxJ0RRCPg0eB-PXsWA

Steps to Reproduce the Bug or Issue

1. Click Run.
2. Outside becomes 1 while Inside remains visible showing 0.
3. After one second, Inside disappears.

Expected behavior

Both paragraphs should show the same latest value while they are visible.

Screenshots or Videos

No response

Platform

Verified with compiled JSX in jsdom on Node.js 24.18.0, in development and production builds.

Additional context

Reproduced on next at a0d6dd2, after #3439 merged. Also reproduces on #3337 at 20f3143.

Possibly related to #3438, but replacing Show with {count() === 0 ? <p>Inside: {latest(count)}</p> : null} avoids this mismatch. Keeping Show mounted with when={true} also makes both values update correctly.

[ryansolid]

Confirmed and fixed in #3450.

Root cause: when a held Show removes its branch, the branch's effects become zombies that keep rendering mainline until the commit disposes them. The flush cancels the recomputes the transaction's own staged writes queued on those zombies (a world they never display) — but the latest() companion is exactly the mainline-visible channel, so the cancel also dropped the Inside update. A zombie dirtied through the lane channel now runs: Inside: 1 | Outside: 1 while the action is pending, then Inside goes at settlement. Your when={true} and inline-conditional variants avoided it because neither produces a zombie.

— Claude via Cursor

[ryansolid]

Fixed on next in 05c7e21 via #3450.

Root cause: when the parking batch is the transaction, flush cancels the zombie recomputes its staged writes queued — a node about to be disposed by the commit renders mainline until then, and the staged world is one it never displays. A zombie dirtied through the lane channel (an override, or the latest() companion the write synced) is the exception: the lane's values are the mainline frame, so the still-visible Show branch kept showing latest(count) at 0 beside the same read outside at 1.

Fix: such a zombie now runs; its pass runs under the lane and lands on the lane's effect queue, so a held lane defers it exactly as it defers every other reader's. 500: Inside: 1 | Outside: 1, 1500: Inside gone. Pinned beside the #3404 case in tests/nested-render-effect-async-cleanup.test.ts.

— Claude via Cursor