Responsive UI — default, not a feature request

The rule

Every UI change ships working at all four device classes. Nobody has to ask.

Device	Test width	Tailwind context
Mobile	375px	base (no prefix)
Tablet	768px	md:
Desktop	1280px	xl:
Large desktop	1920px	beyond 2xl:

Tailwind is mobile-first: unprefixed classes are the mobile layout; prefixes add behavior at wider screens (sm: 640, md: 768, lg: 1024, xl: 1280, 2xl: 1536). Write the mobile layout first, then widen — not the other way around.

A change is NOT done until you have reasoned through (or better, viewed via the webapp-testing skill / browser dev tools) what it looks like at 375, 768, 1280, and 1920. If you only checked one width, you checked none.

Works WITH the design system — precedence rules

This skill layers on top of the ui skill; where they touch, the design-system rules win. Responsiveness is achieved THROUGH the DS, never around it:

• Never put responsive classes (or any className) on DS components (Text, Stack, HStack, Badge, Button, …) — they don't accept it. Put breakpoint utilities on a wrapper <div> (the ui skill's "Layout with Wrapper Divs" pattern):

  // ✅ breakpoints on the wrapper, DS component untouched
  <div className="hidden sm:block"><Badge variant="outline">Active</Badge></div>
  // ❌ className on a DS component
  <Badge className="hidden sm:block">Active</Badge>
  

• Reach for DS layout primitives first (PageLayout, Stack, HStack, Section) — they carry responsive spacing already. Raw responsive divs are for what the primitives don't cover, not a replacement for them.
• Arbitrary pixel values stay an anti-pattern (w-[847px] ❌ — per the ui skill). Prefer the standard Tailwind scale (max-w-sm, w-48, max-w-xs). A fixed control width like w-[200px] is acceptable ONLY when it matches an established pattern already used on that surface, and it must still carry a responsive strategy (see below).

Non-negotiables

1. The page body never scrolls horizontally. Wide content (tables, code, diagrams) scrolls inside its own container, never the page.
2. No fixed width without a responsive strategy. A fixed-width control is only acceptable if the element hides, shrinks, or wraps on smaller screens (hidden sm:block, w-full md:max-w-xs, or a wrapping parent) — with the breakpoint classes on a wrapper div, never on a DS component.
3. Touch targets on mobile: interactive elements ≥40px tall; don't rely on hover for anything essential (no hover on touch devices).
4. Test with real content lengths — long names, emails, 33-item counts. Use truncate / min-w-0 on flex children that hold text.
5. Large desktops: content must not stretch into unreadable full-width lines — cap with max-w-* containers where the layout doesn't already.

Repo patterns (use these, don't invent)

• Tables: the design-system Table already wraps rows in overflow-x-auto — wide tables scroll horizontally inside themselves on mobile. That is the accepted pattern for dense admin tables. Do NOT try to reflow table columns per breakpoint; do keep cells reasonable (min-w-* max-w-* on cell content is fine).
• Toolbars / filter rows: primary control (search) is w-full md:max-w-[300px]; secondary controls (filters, source selectors) collapse on phones with hidden sm:block. Example: the People tab toolbar in apps/app/.../people/all/components/TeamMembersClient.tsx.
• Page shells: use design-system PageLayout / Stack / HStack — they carry the responsive spacing; don't rebuild them with raw divs.
• Grids: grid-cols-1 md:grid-cols-2 xl:grid-cols-3 style progressions — never a fixed column count for all widths.
• Sheets/Modals: design-system Sheet/Dialog are responsive already; don't fix their widths with arbitrary values.
• Images/logos: constrain (max-w-full, explicit small sizes); never let an image force layout width.

Checklist before "done" (run mentally or in the browser)

• 375px — nothing overflows the viewport; controls usable or intentionally hidden; text truncates instead of breaking layout
• 768px — secondary controls visible; layout uses available width sensibly
• 1280px — the intended "design" width; matches mockups
• 1920px — no absurdly stretched content; whitespace balanced
• Long content (names, emails, high counts) doesn't break any of the above
• Tests: if a component hides/reflows per breakpoint in a way that matters, its test asserts the class contract (e.g. hidden sm:block)

When you can skip a width

Admin-only dense screens (e.g. internal tooling) may treat mobile as "usable via horizontal scroll" rather than fully reflowed — but that must be a conscious choice following the existing pattern of that page, never an accident of not checking.