fix(runner): mount the preview once when npm latest is ahead of the buckets (DEV-3344)

[demtario]

Context

The o11y local integration workflow (e2e-o11y-local.yml) has been red on master since 2026-10-07. telemetry-metrics.spec.ts:203 gets two preview.ready_ms points instead of one, and o11y-local.spec.ts intermittently reads outcome: "abandoned".

No commit caused it: between the last green run and the first red one, the only change was docs-example JSON. A visit with no version in the URL starts on DEFAULT_VERSION, the newest committed bucket pin (18.1.1), and switches to npm latest once /api/versions answers. The preview mount did not wait for that answer. When handsontable@18.1.2 was published ahead of the weekly bucket re-pin, every such visit booted the preview on 18.1.1, tore it down, and booted it again on 18.1.2. That cost two Sandpack boots (or two container sessions on Tier-2) and sent one abandoned point. The intermittent o11y-local failure is the same race: it passes when /api/versions answers before the starter artifact loads.

This recurs after every npm release until the Monday import-starters re-pin. The re-pin will turn master green on its own, but the bug returns with the next release.

Fix: versionPending in App.tsx now also holds the mount while a visit that pinned nothing is still on the DEFAULT_VERSION placeholder and /api/versions has not answered. Only the mount waits; the starter artifact still loads in parallel. Saved demos, ?v= links and docs routes are unchanged. Trade-off: if /api/versions is slow, an unpinned visit shows booting until it answers (worst case one 5 s timeout), then mounts on the fallback.

Types of changes

• New example
• Update to an existing example
• README / documentation change
• Demo runner (runner/) change
• CI / tooling change

How was this verified?

• Cause, A/B on one local build with only latest varied: stubbing it to 18.1.1 made telemetry-metrics Tier-1 pass; the real 18.1.2 made it fail with Expected: 1, Received: 2.
• New deterministic test in telemetry-faro.spec.ts (runs per PR in ci.yml's e2e-telemetry job). It holds /api/versions until the starter has loaded, then asserts that no abandoned preview.ready_ms point is sent. It fails 3/3 on the previous App.tsx and passes with the fix.
• e2e-o11y-local.yml dispatched on this branch and on master at the same time, while npm latest (18.1.2) still differed from the bucket pin (18.1.1). Branch: both jobs green, 2/2 tests each, no retries (run 37639930705). Master: telemetry-metrics still red with 1 vs 2 points (run 37639936182). That workflow's path filter does not match this diff, so it will not run on the PR by itself.
• Locally: live telemetry-metrics Tier-1 + Tier-2 green; telemetry-faro + example-analytics 30/30; full deterministic suite 247 passed, 0 failed, 1 flaky. The flaky test, style-panel.spec.ts:317 (file-tree row after reload, not the preview mount), passed 15/15 when run on its own. pnpm typecheck clean.

Checklist

• New/renamed example: added to runner/config/frameworks.json (see CONTRIBUTING.md); otherwise it won't appear on demos.handsontable.com (n/a: no example change)
• New example: added a row to the tables in README.md (n/a)
• Ran pnpm build (and pnpm dev) in the affected example/server-example locally (n/a: runner-only change; ran pnpm build + the e2e suites in runner/ instead)

Related issue(s):

1. DEV-3344

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Note

Medium Risk
Changes when the preview first mounts for unpinned visits and could briefly delay booting if /api/versions is slow; pinned URLs and saved demos are unaffected.

Overview
Fixes DEV-3344: unpinned visits no longer boot Sandpack twice when npm latest is ahead of the committed bucket default.

versionPending in App.tsx now stays true for bare visits still on DEFAULT_VERSION until /api/versions resolves, so the preview mount waits for the swap to npm latest. Starter loading is unchanged and still runs in parallel; only the runtime mount is deferred (worst case: booting until the versions fetch finishes).

Adds a deterministic Playwright case in telemetry-faro.spec.ts that delays /api/versions until the starter pins on the default, then checks there are no preview.ready_ms measurements with hot.outcome: "abandoned".

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