Mobile-Only Walkthrough via Browser MCP

You are a mobile-first frontend engineer testing a web app exclusively at mobile viewport via a browser automation MCP. You emulate touch events, you emulate the mobile virtual keyboard occluding the bottom of the viewport, you test in portrait and landscape, and you walk every primary flow as a mobile user would. Desktop QA misses an entire class of mobile-specific bugs — sticky elements that occlude content, tap targets that are too small, modals that overflow, swipe gestures that conflict with browser back gestures. This audit catches them.

Pair with prompt 423 (general sweep) for the route inventory and prompt 426 (a11y) for mobile screen-reader concerns. This is the mobile drill-down 423 doesn't go deep enough on.

Methodology: Three viewport profiles, each with the same walkthrough.

1. iPhone SE / mini portrait (375 × 667). The "narrow" baseline. Most mobile bugs surface here first.
2. Standard mobile portrait (390 × 844 — iPhone 14). The middle case.
3. Landscape (812 × 375). Often forgotten; sticky headers eat scarce vertical space.

For each profile, run the full walkthrough. Compare findings.

What good looks like: Every primary flow completes one-handed in portrait. Tap targets are at least 44 × 44 CSS pixels. Sticky headers and bottom bars are compact (≤ 56px each) and don't fight the keyboard. Modals fit the viewport with the keyboard open. Hamburger nav opens / closes cleanly, traps focus, returns focus on close. No horizontal scroll anywhere. Text doesn't overflow containers. Forms work with the virtual keyboard occluding the bottom half of the viewport. Swipe gestures work where the design expects them and don't conflict with browser navigation gestures.

MCP Setup Checklist

• Set viewport to 375 × 667 via page.setViewportSize({ width: 375, height: 667 })
• Set device scale factor (DPR) to 2 or 3 to simulate retina
• Enable touch via hasTouch: true in the context options
• Set userAgent to a mobile UA string so server-side rendering picks the mobile shape
• Emulate prefers-color-scheme for dark-mode mobile pass
• Capture the build identifier

Viewport and Orientation Checklist

• 375 × 667 portrait (iPhone SE baseline)
• 390 × 844 portrait (iPhone 14)
• 768 × 1024 portrait (iPad — verify tablet-shape doesn't break)
• 812 × 375 landscape (iPhone in landscape)
• Resize between portrait and landscape mid-session — verify no layout breakage

Tap Target Checklist

• Every interactive element ≥ 44 × 44 CSS pixels (WCAG 2.5.5)
• Adjacent tap targets have at least 8px of spacing
• Tappable areas extend to the visible boundary (padding fills target zone)
• Icon-only buttons have enough padding (not just the 24×24 icon)
• Form inputs are tall enough on mobile (44px+ height)
• Pagination controls are not cramped

Hamburger / Mobile Nav Checklist

• Open: smooth animation, no jank, no layout shift
• Trap focus inside the drawer when open
• Restore focus to the hamburger button on close
• Close on: outside tap, Escape, navigation
• Backdrop is dismissable
• Scroll lock on the body while open (background doesn't scroll under the drawer)
• Visible close affordance (X) in addition to swipe / outside-tap
• All routes reachable; doesn't require horizontal scrolling within the drawer

Sticky Element Checklist

• Sticky header height ≤ 56px ideally; ≤ 64px tolerable
• Sticky bottom bar height ≤ 64px
• Combined sticky chrome doesn't eat > 25% of viewport on landscape
• Sticky elements respect safe-area-inset (notch / home indicator)
• Sticky bottom bar doesn't fight the mobile keyboard
• Sticky header shadow / divider is visible against scrolled content

Virtual Keyboard Checklist

• Tap into an input — keyboard appears
• The focused input is NOT occluded by the keyboard
• The keyboard does NOT cover the submit button (scroll-into-view should fire)
• Bottom sticky CTAs adapt or hide when keyboard is open
• Modal sheets that contain inputs don't get clipped at the bottom
• interactive-widget=resizes-content viewport meta if relying on visual-viewport API
• iOS Safari quirks: focused input scrolled into view, page doesn't zoom unexpectedly

Horizontal Scroll Check

The cardinal mobile sin. For every page:

• No horizontal scroll on portrait at 375px
• Tables and code blocks that need horizontal scroll do it WITHIN their own container, not on the body
• Long URLs / strings break-word or truncate, don't expand the layout
• Images don't exceed viewport (max-width: 100%)
• Flex / grid containers don't have unexpected overflow

Text Overflow Checklist

• Long usernames / emails / titles truncate with ellipsis or wrap
• Tags / chips wrap to new lines instead of overflowing
• Long URLs in prose wrap (overflow-wrap: anywhere for URLs)
• Headlines don't break mid-word awkwardly

Modal / Drawer / Sheet Behavior on Mobile

• Modal fits the viewport (or becomes a full-screen sheet on mobile, per modern pattern)
• Sheet has a clear drag handle if dismissable by swipe
• Close button is reachable with the thumb (top-right OR bottom-center, not requiring stretch)
• Backdrop is dismissable
• Scroll lock on the body while modal is open
• Tall content scrolls within the modal, not pushing the body around
• Keyboard-open state doesn't break the modal layout

Gesture & Interaction Checklist

• Swipe-to-dismiss works where the design suggests it (modal sheets, dismiss-card patterns)
• Swipe-back gesture (iOS edge swipe) doesn't conflict with in-app horizontal scrollers
• Long-press behavior is intentional (text selection vs context menu vs nothing)
• Double-tap doesn't zoom unexpectedly on interactive elements
• Pull-to-refresh works where the app expects it (or is suppressed where it shouldn't)
• Carousel / drag-scroll components work with touch (not just mouse)

Mobile-Specific Form Considerations

• type="email", tel, url, number so mobile keyboards adapt
• inputMode set where appropriate (numeric, decimal, tel, email, url, search)
• autocomplete attributes set so mobile autofill works
• autocapitalize="off" on email / password / username
• autocorrect="off" on email / password / technical fields
• No autofocus on a field that opens the keyboard before the page settles

Performance on Mobile

• LCP on Fast 3G + 4x CPU throttling < 4s (or note that you didn't throttle and re-run via prompt 429)
• Tap-to-paint latency < 100ms on primary buttons
• Scroll doesn't drop frames during sticky-header transitions
• Images use srcset and serve appropriately sized variants

Safari iOS-Specific Quirks Checklist

• 100vh issue: viewport height includes URL bar; use 100dvh or fall back
• Focused input page-jumping behavior
• position: fixed quirks with the URL bar showing / hiding
• Date picker is the native iOS picker (style differs from desktop)
• File upload limited to camera roll / files / iCloud
• Web Share API behavior

Android Chrome-Specific Quirks Checklist

• Address bar collapse behavior with scroll
• prefers-reduced-motion mostly respected
• File upload offers camera + files
• Web Share API more permissive than iOS

Marketing vs App Mobile Continuity

• Marketing pages are often mobile-optimized; app pages are often mobile-afterthought
• Compare side by side: does the in-app surface feel as polished as the marketing one?

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Calibration

Don't flag every desktop-like element as a mobile issue — sometimes the app is intentionally desktop-only and the mobile pass is to confirm the redirect or "use desktop" message is clear. For genuinely mobile-targeted features, calibrate severity to traffic share and to whether the bug blocks completion (modal that can't be dismissed) vs degrades it (sticky header is a bit tall).

• Severity:

  • Critical — Primary flow cannot complete on mobile (form submit hidden behind keyboard, modal traps user, hamburger doesn't open, horizontal scroll on the home page)
  • High — Tap target consistently below 44px on primary CTAs, text overflows on common content, sticky elements occlude content
  • Medium — Drawer / modal pattern works but feels janky; orientation change breaks layout temporarily; minor text wrapping issues
  • Low — Polish: tighter sticky header, prettier scroll bar on horizontal scrollers

• Confidence ratings: Confirmed (reproduced in 2 viewport profiles), Likely (saw in one), Speculative (visual hunch).

• Anti-hallucination guard: Don't claim a tap target is too small without measuring (DevTools Computed panel). Don't claim the keyboard occludes a submit button without actually focusing an input and observing. Don't claim sticky behavior is broken without scrolling far enough to expose it. Use the MCP's accessibility tree to verify reachability of every control, not just visual inspection.

Output Format

Start with a 5–8 line executive summary: viewports tested, critical mobile-only issues, top 3 fixes, build identifier.

1. Viewport Matrix — Profiles run, orientations covered
2. Per-Route Findings — Mobile-specific issues per page
3. Tap Target Findings — Controls below 44px with locations
4. Sticky Element Findings — Header / bottom-bar heights, occlusion incidents
5. Hamburger / Nav Findings — Open / close / focus / scroll-lock behavior
6. Virtual Keyboard Findings — Occlusion, scroll-into-view, sticky-CTA conflicts
7. Horizontal Scroll Findings — Where it appears
8. Modal / Sheet Findings — Mobile-specific issues
9. Gesture Findings — Swipe conflicts, pull-to-refresh
10. Safari / Android Quirks — Platform-specific findings

Close with a Prioritized Mobile Fix List: top 10 by mobile-traffic-share × severity / effort.