Fix calculator navigation flash with persistent layout

[SakshiKekre]

Context

Navigating between calculator pages (reports, policies, simulations, households) caused a visible white flash because each page mounted its own StandardLayout and CalculatorProviders. Next.js unmounted the entire component tree on every navigation, destroying and rebuilding the sidebar/header DOM.

This was tested across 4 approaches on Vercel preview deployments with a perf instrumentation harness (PRs #915-#918). See #917 for detailed benchmarks and comparison data.

Summary

Navigation flash fix

• Move StandardLayout + CalculatorProviders into [countryId]/layout.tsx so they persist across all page navigations
• Add FullScreenPortal component — the parameter selector renders as a fixed full-screen overlay via React portal, covering the sidebar/header naturally
• Pathway wrappers no longer conditionally wrap/unwrap StandardLayout — full-screen views use <FullScreenPortal> instead
• Strip CalculatorProviders and StandardLayout from all individual page files

API error handling fix

• Wrap SocietyWideCalcStrategy.execute() in try/catch — returns error CalcStatus instead of throwing (matches HouseholdCalcStrategy pattern). Previously, a 500 from the Modal gateway would propagate as an uncaught async error and corrupt the Next.js router state, freezing all internal navigation.
• Remove throw error from CalcOrchestratorManager.startCalculation() catch block — logs and cleans up without re-throwing
• Update test to match new behavior (error is swallowed, not re-thrown)

Perf results (from #917 preview testing)

• Listing↔listing (cached): 3-6ms (previously 820-1400ms with remounts)
• Listing→create (first load): 874-1488ms (chunk download, same as before)
• Param selector enter/exit: instant mode change, no mount/unmount
• Sidebar/StandardLayout/CalculatorProviders: mount once at page load, never unmount

Test plan

• Navigate between listing pages (reports, simulations, policies, households) — no white flash
• Navigate listing → create pathway — sidebar stays, content swaps
• policies/create → parameter selector — full-screen overlay, no double header
• Back from parameter selector — sidebar reappears instantly
• Same flow for simulations/create → parameter selector
• Report builder and report output pages render correctly
• Trigger a report with a failing API — sidebar nav should remain functional

[vercel]

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Project	Deployment	Actions	Updated (UTC)
policyengine-app-v2	Ready	Preview, Comment	Apr 15, 2026 6:55pm
policyengine-calculator	Ready	Preview, Comment	Apr 15, 2026 6:55pm
policyengine-calculator-next	Ready	Preview, Comment	Apr 15, 2026 6:55pm
policyengine-website	Ready	Preview, Comment	Apr 15, 2026 6:55pm