PPR capture: outlined-but-ready Suspense content postpones — counted-abort gate needed (large sync sections re-render per HIT) #702

Problem

Fizz OUTLINES any Suspense boundary over ~500 bytes (react-dom isEligibleForOutlining), and in a tracked-postpones prerender renderSuspenseBoundary writes the FALLBACK inline first with the content as a queued task. The capture abort (quiesce → payloadSettled → 16 task hops) can land before that content flushes, postponing boundaries whose content was fully renderable with zero pending data. Every HIT then re-renders and re-streams those sections.

Measured on a production storefront homepage (2026-07-06): ~100 postponed boundaries, ~2.7MB of resume HTML regenerated per request at ~5MB/s workerd throughput — the entire post-shell latency tail (~400ms). App-side workaround shipped there (drop <Suspense> around plain values), but any consumer wrapping large synchronous content in Suspense hits this silently.

What was tried and disproven

A work-quiet abort gate (grant task turns while inter-hop timer gaps show work; abort after K idle hops): on the red reproducer the gate observed 16/16 idle hops while the capture stored a 4.5KB prelude — fizz was not queued behind task turns, it was suspended (the fixture's 30k-row section serializes as one huge flight row; fizz blocked on the row thenable, and the flight-gate/SSR interplay for oversized single rows needs its own look). Timer-gap sensing cannot distinguish "blocked on masked data" (correct to abort) from "blocked on capture-machinery starvation" (wrong to abort).

Direction

Counted/tracked-abort semantics instead of physics:

• Next.js: classic PPR has NO abort — dynamic-ness is a synchronous unstable_postpone signal and the prerender runs to natural completion, so outlined content always flushes. cacheComponents mode uses a two-phase render with a refcount signal (CacheSignal, incl. tracked import()s — PR [dynamicIO] cache tracking for import() vercel/next.js#74152) and aborts at a fixed task boundary only after the count drains.
• vinext (cloudflare/vinext): explicit pending-task counting + an epoch guard against late settles.
• For rango: the capture knows its masked promises (loader masks, nested-thenable masks). Everything else pending should be allowed to settle/flush before abort — i.e., abort when (pending − masked) = 0 at a task boundary, deadline-bounded. That makes outlined-but-ready content bake by construction and is the structural successor to the quiet-window + payload-settled patches (fix(vite): prerender collection ran trieless — catch-all shadowed nested routes; Prerender + ppr composition coverage #697).

Red reproducer (in-tree, test.fixme)

packages/rangojs-router/e2e/shell-cache.test.ts — "large sync Suspense section bakes into the shell" + the /shell-cache/outlined fixture (e2e/test-app/src/urls/shell-cache.tsx): a 30k-row synchronous Suspense section next to a normal loader hole; asserts the rows arrive in the same flush window as the first byte of a HIT. Currently red: the capture stores a 4.5KB prelude and ~20MB streams per request. Enable with the fix. Note the fixture also exposes the single-giant-flight-row interaction — worth splitting into a many-medium-boundaries variant (the storefront shape) when picking this up.

Sequencing

After #701 (producer B) lands — same files (ssr/index.tsx, shell-capture.ts, rsc-rendering.ts), and build-time captures benefit from the same gate.

References

• fix(vite): prerender collection ran trieless — catch-all shadowed nested routes; Prerender + ppr composition coverage #697 (payload-settled gate — the reactive patch for one instance of this class), feat(ppr): shell fast path — HIT tails replay the handler layer; only loaders are dynamic #695, PPR fast path: emit stored snapshot fragments verbatim into the HIT payload (stop re-serializing baked segments per request) #700, feat(ppr): producer B — emit the shell entry at build time for prerendered routes (#699) #701.
• Framework research memos (2026-07-06 session): Next.js prerender/postpone internals, vinext counted-abort, Waku slot model.