Layout Mechanics

Style skills decide how a surface looks. This file decides how it is built to hold content — what scrolls, what stays fixed, how regions shrink, and which layouts survive real data. It stacks on top of any Layer A style skill (taste-skill, minimalist-skill, soft-skill, brutalist-skill, gpt-tasteskill) and any Layer B brand. It adds ZERO visual direction — color, type, shadow, radius, and motion still come from the style skill and DESIGN.md.

Load this when the screen is an application shell, not a scroll-the-whole-document marketing page: dashboards, settings, list-detail, mail/inbox, command surfaces, split panes, sidebars — or when a page that looked fine breaks the moment content gets long, empty, or unbroken.

1. Scroll ownership — decide this BEFORE writing layout CSS

The single most common agent-built layout bug is an app shell where the wrong thing scrolls: the whole page scrolls when only a panel should, two panels fight over the scrollbar, or a "fixed" header scrolls away. Prevent it by naming ownership up front, in DESIGN.md and in the component:

• What scrolls? Name the ONE element that owns vertical scroll for this region.
• What stays fixed? Header, sidebar, footer, toolbar — list them.
• Where is height determined? The scroll container needs a bounded height ancestor, or it will grow instead of scroll.

Rules:

• One scroll container per region unless each extra one has a named job. Nested scrollbars with no declared responsibility are a defect — the user loses track of what a wheel/trackpad gesture will move.
• Full-height shells are bounded by 100dvh/100dvb (dynamic viewport units), never 100vh — vh causes the iOS Safari address-bar jump.
• A sticky element (position: sticky) follows document scroll; a fixed shell region (grid row/column with overflow: auto body) owns its own scroll. Do not mix the two models in one region without reason.

2. The two CSS contracts agents get wrong

These two are worth memorizing verbatim. They fail silently — the layout looks right until content arrives.

Bounded scroll shell (fixed header/footer, scrolling body):

.shell {
  display: grid;
  grid-template-rows: auto minmax(0, 1fr) auto; /* header / body / footer */
  max-block-size: 100dvb;
}
.shell__body {
  min-block-size: 0; /* WITHOUT THIS the grid child refuses to shrink and overflow never fires */
  overflow: auto;
}

The min-block-size: 0 (or min-height: 0) on the scroll child is the fix for "why won't my panel scroll — it just pushes the footer off-screen." A grid/flex child's default min-*-size: auto refuses to shrink below its content. The same applies to a flex column: the scrollable child needs min-height: 0.

Overflow-safe intrinsic grid (repeat as many columns as fit, no media queries):

.grid {
  display: grid;
  gap: var(--gap);
  grid-template-columns: repeat(auto-fit, minmax(min(16rem, 100%), 1fr));
}

The inner min(16rem, 100%) is load-bearing: plain minmax(16rem, 1fr) forces a 16rem track even when the container is 12rem wide, causing horizontal overflow on narrow screens. min(…, 100%) lets the track collapse below its floor when space is genuinely tight. Use auto-fit to stretch the last row, auto-fill to keep empty tracks.

3. Named layout primitives (shared vocabulary)

Build shells from these named primitives instead of ad-hoc fl/grid. Naming them makes DESIGN.md Section 5 and subagent handoffs precise ("wrap it in a sidebar with a 20rem aside" beats "put it on the left, roughly"). Lineage: Every Layout + web.dev one-line layouts.

4. Container-local vs viewport-level responsiveness

Ask what the layout is responding to before reaching for a breakpoint:

• The component should adapt to its own available width (a card that's wide in main but narrow in a rail) → wrap it in container-type: inline-size and use @container. This is correct far more often than agents assume, because a component rarely knows the viewport it lands in.
• The page frame itself changes (sidebar collapses, columns drop) → @media.

Prefer intrinsic adaptation (switcher, sidebar, intrinsic grid, clamp()) over any query — the best breakpoint is often none. Use breakpoint names for layout states, never device names (--bp-wide, not --bp-ipad).

5. Recipes are spatial models, not product categories

Choose a layout by the screen's spatial shape, not its product label. A settings page and a docs app both want fixed-sidenav-shell; a support inbox and a file browser both want list-detail; a metrics view and a photo gallery both want an intrinsic grid. Do not invent a bespoke "dashboard layout" when page-grid + intrinsic grid + cluster already covers it — and do not force marketing-page structure (hero, zigzag, bento) onto a task app.

6. Content stress — the layout is not done until it survives this

Landing pages fail on taste; app shells fail on content. Before declaring any layout done, stress every region against:

• Empty — no rows, no avatar, no value. Does the region collapse gracefully or leave a broken frame?
• Long label — a 40-char name in a 12-char slot. Truncate (text-overflow: ellipsis) or wrap by design, never by accident.
• Long paragraph — does the measure stay readable, or does text run 200 chars wide?
• Unbroken string — a URL or token with no spaces. Needs overflow-wrap: anywhere / min-inline-size: 0, or it forces horizontal scroll.
• Reflow — at 375px width the layout reflows to a single readable column with NO horizontal scrollbar. Two-dimensional scrolling of primary content is a fail.
• Direction — if the app supports RTL, the layout uses logical properties (margin-inline, inset-inline-start) so it mirrors correctly.

A layout that only holds the happy-path mock is not finished. Drive these states in /visual-qa alongside the interaction states the style skill requires.

Boundary

This file owns spatial structure only. It never sets color, typography, shadow, radius, or motion values — those trace to DESIGN.md and the loaded style skill. If you find yourself adding a brand color to a layout primitive, stop: the primitive stays layout-only and the styling wraps or composes around it.