Site tier-1: open-set banner, sensitivity selector, bootstrap intervals

[MaxGhenis]

Summary

Implements the tier-1 leaderboard improvements from docs/site_improvements_scope.md, plus a shared sticky header that the /paper page now reuses (matching the home page chrome, without the Global/US/UK view selector). Round-2 review feedback is incorporated: the bootstrap estimator now matches the canonical row-equal scoring rule, and the global view auto-falls-back to Main when an active sensitivity slice has no rows in one country.

Header

• Extract SiteHeader from Hero. The new component owns the sticky brand + nav + view-selector + action-link layout and supports an alwaysExpanded mode for pages that don't drive their own collapse.
• Hero refactored to wrap SiteHeader and pass the country-aware subtitle, stat strip, and snapshot pill as expandedContent. Drops the Top score stat and the Leading: <model> sidebar — the leaderboard itself is the canonical source for both.
• /paper uses SiteHeader with alwaysExpanded, no view selector, and a Benchmark action link. The page body keeps its eyebrow/buttons/iframe; the inline H1 is gone since the header carries the brand.
• Picked up the upstream parallel design tweak: dropped the inline "by [PE logo]" tagline next to the title and added a persistent "by PolicyEngine" pill at the right edge of the header.
• Collapsed nav items and action link are no longer keyboard-focusable while hidden (tabIndex={-1}, aria-hidden); the scroll listener short-circuits on alwaysExpanded.

Open-set banner and snapshot pill

• role="note" warning-tinted banner above the leaderboard.
• Snapshot date pill (Snapshot 2026-05-01) on the home stat row and next to the Manuscript eyebrow on /paper.

Sensitivity-view selector

• Segmented control: Main / Amount only / Binary only / Positive cases / Zero cases. Selecting a view rescores models client-side from scenarioPredictions and reorders the table; the description for the active view appears inline. The control is role="group" with aria-labelledby; each button has aria-pressed.
• Auto-fallback for global: when a slice has no rows in one country (e.g. Binary-only on Global, since UK has zero binary outputs), viewSupportsGlobal returns false, the leaderboard falls back to the Main view with a role="note" notice, and the offending button becomes aria-disabled with the click handler short-circuited so it cannot simultaneously be pressed and disabled.
• New utilities under app/src/lib/:
  • scoring.ts ports score_single_prediction (mean of exact / within-1% / within-5% / within-10% for amount outputs; classification accuracy for binary; output-group resolution for person-expanded variables). Verified against canonical analysis.py for both countries' top models.
  • sensitivity.ts builds the per-row score table from a DashboardBundle and aggregates output-group means → country → global, preserving country-equal weighting. Exposes viewSupportsGlobal for global-validity checks.

Bootstrap rank intervals

• bootstrap.ts implements household-resampling with a deterministic mulberry32 RNG (seed 42, DEFAULT_DRAWS = 400). Inside each draw it adds bucket sum and count directly across sampled scenarios so each row contributes equally to the output-group mean — matching the headline modelScoresForView estimator instead of collapsing each scenario to a per-scenario mean first.
• ModelLeaderboard renders Rank N(-M) · 95% L-U next to each model's point estimate. Sample current Global view: Rank 1 has 95% CI ~74.7–79.6; Rank 2-3 cluster ~73.5–78.7. Tooltip names the bootstrap parameters.
• Snapshot top-3 scores against current app/src/data.json: US gpt-5.5 90.03 / grok-4.20 89.29 / gemini-3.1-pro-preview 88.21; UK gpt-5.5 77.18 / gemini-3.1-pro-preview 76.18 / grok-4.20 75.07; Global gpt-5.5 83.60 / gemini-3.1-pro-preview 82.20 / grok-4.20 82.18.

Repo

• .gitignore keeps the Python lib/ blanket ignore and adds an explicit !app/src/lib/ + !app/src/lib/** allowlist so app/src/lib/ is tracked while nested lib/ directories elsewhere stay ignored.

Verification

• bun run lint — clean
• bun run build — clean (Next.js 16 production build)
• bun run start — SSR HTML on / contains the open-set banner with role="note", the snapshot pill, the sensitivity selector with aria-pressed × 5 and role="group", the country view selector with aria-pressed × 3 and role="group", per-model bootstrap intervals, and the "by PolicyEngine" pill. /paper renders the same chrome without the view selector.
• Scoring math reconciled against policybench/analysis.py for the snapshot's top-5 models per country.

Test plan

• CI passes
• Visit / — confirm header drops "Top score" / "Leading" and the snapshot pill is visible
• Switch sensitivity view to Positive cases → leaderboard reorders and intervals refresh
• Switch to Binary only while on Global → leaderboard falls back to Main with a notice; the Binary-only button is aria-disabled
• Visit /paper → same sticky header style without a view selector
• Mobile width → bootstrap interval line wraps under each model row

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Project	Deployment	Actions	Updated (UTC)
policybench	Ready	Preview, Comment	May 6, 2026 2:30pm