Thumb-First · Platform

The technical-verification layer of the thumb-first suite. Where thumb-first-design decides what the mobile design should be (platform-agnostic judgment), this skill verifies that the implementation holds up on the actual device — objective defects with file:line and fixes, rated by severity.

Run /thumb-first for a combined design + platform review; use this skill directly for technical checks only.

Step 0 · Detect the Target (do this first)

The relevant defects are completely different for a web/PWA vs. a native app. Determine the target before doing anything else.

Sniff the repo:

• Web / PWA → next.config., vite.config., an index.html, a web app manifest.json/manifest.webmanifest, a service worker, Tailwind/CSS. This is this skill's home turf — proceed to the principles and routing below.
• Native → react-native/expo in package.json, app.json/app.config. (Expo), an ios/+android/ pair, .xcodeproj/Package.swift/*.swift (SwiftUI/UIKit), or pubspec.yaml (Flutter).
• Ambiguous or both (e.g. an Expo app with a web target) → ask which target to audit.

If the target is NATIVE — do not fake a web audit. This skill's checks (Safari quirks, viewport units, service workers, web manifest) do not apply. Instead:

1. Check for an installed native skill to hand off to — scan available skills / ~/.claude/skills/ for ones describing React Native, Expo, SwiftUI, native iOS/Android, or Flutter (e.g. react-native-design, building-native-ui, vercel-react-native-skills).
2. If one exists → hand off. Name it explicitly: "For the native technical pass, use <skill> — it covers native performance, lists, and platform APIs." Note that thumb-first-design still covers the design judgment for native, since that layer is platform-agnostic.
3. If none exists → say so plainly. Do not overreach:

> thumb-first-platform covers mobile design judgment (via thumb-first-design) and web/PWA technical defects. It is not set up to run native-platform (React Native / SwiftUI / Flutter) performance or technical audits. For that, install a native-specific skill, or use platform-native tooling — Xcode Instruments, Android Studio Profiler, Flipper, or the React Native performance monitor.

The design layer applies to every target. This technical layer is web/PWA-deep and native-aware-but-delegating. Be honest about that boundary.

Mobile Web & PWA — Core Principles

1. iOS Safari is the problem child

Most quirks and limitations live here. Always test on real iOS devices — simulators miss critical behaviors. Key pain points:

• PWAs require Safari for installation (Add to Home Screen)
• Push notifications only work when installed to home screen (iOS 16.4+)
• 7-day storage cap for non-installed web apps; 50MB cache limit; no background sync
• WebKit-only rendering (even Chrome on iOS uses WebKit)

2. Viewport units matter

Never use 100vh for full-height layouts on mobile — browser chrome causes overflow.

.hero  { height: 100vh; height: 100svh; }  /* svh for static elements (90% of cases) */
.menu  { height: 100dvh; }                 /* dvh only for elements that should resize */

dvh causes layout shifts — use sparingly.

3. Safe-area insets are required

Notches, Dynamic Islands, and home indicators are everywhere.

<meta name="viewport" content="width=device-width, initial-scale=1.0, viewport-fit=cover">

.header     { padding-top: max(1rem, env(safe-area-inset-top)); }
.bottom-nav { padding-bottom: max(1rem, env(safe-area-inset-bottom)); }

4. Touch targets ≥ 44px

Apple HIG requires 44×44pt minimum; Google recommends 48×48dp. Never go smaller for tap targets.

5. Test on real devices

Chrome DevTools emulation lies — it doesn't simulate address-bar behavior, safe-area insets, touch-delay nuances, iOS Safari CSS bugs, or real mobile-hardware performance. Use real devices or BrowserStack/Sauce Labs.

Intake & Routing

What do you want to do?

1. Audit an existing mobile app/PWA for issues
2. Fix iOS Safari–specific problems
3. Set up PWA (manifest, service worker, icons)
4. Optimize mobile performance (Core Web Vitals)
5. Debug touch/gesture issues
6. Set up push notifications
7. Test mobile compatibility

Wait for the response, then read the matching workflow and follow it.

Intent-based routing (specific problem described):

• "100vh not working", "viewport", "height issue", "notch", "safe area", "dynamic island" → references/viewport-layout.md
• "images too big", "slow images" → references/image-optimization.md
• "rubber band", "bounce scroll" → references/ios-safari-quirks.md

When run via /thumb-first: the umbrella invokes the audit path only (workflows/audit-mobile-pwa.md) and merges its defects into the combined report. The setup/push/perf workflows are build tasks — available on direct invocation, but outside the one-stop review.

Verification Loop — After Every Change

1. Build succeeds? pnpm build
2. Lighthouse audit? Chrome DevTools → Lighthouse → Mobile + PWA
3. Real iOS device? Safari Web Inspector, or ngrok/localtunnel
4. Safe areas handled? Portrait and landscape, with and without notch

Report findings as: Build: ✓ · Lighthouse PWA: X/100 · iOS Safari: [specific issues].

Reference Index

All in references/:

Workflow Index

Common Issues & Quick Fixes

/* 100vh overflow on mobile */     height: 100svh;            /* or min-height */
/* 300ms tap delay */              button, a { touch-action: manipulation; }
/* rubber-band scroll */           html { overscroll-behavior: none; }

<!-- safe area not working -->            <meta name="viewport" content="..., viewport-fit=cover">
<!-- iOS status bar style -->             <meta name="apple-mobile-web-app-status-bar-style" content="black-translucent">

PWA not installable: valid manifest.json · HTTPS · service worker registered · icons include 192×192 and 512×512.