Author a block that looks good in any pane

A layout block doesn't live in a known screen — it lives in whatever pane the user dragged it to. So it must respond to its container, not the viewport, and it must declare how small it's allowed to get. This skill is the playbook; the contract it fills in is read by the viability metric (checkLayoutViability, #710) and the deterministic layout pass (#709).

Part of the layout engine (epic #703). A block is registered in croutonLayoutBlocks (see crouton-core CLAUDE.md, #704) — this skill is about how to build the component behind that registration.

The one hard rule: container queries, not viewport media queries

A block's width ≠ the viewport width. A list in a 280px sidebar on a 4K monitor is narrow, even though the viewport is huge. So:

• ✅ Use @container (Tailwind / Nuxt UI 4 container queries) keyed off a container on the block root. Put @container on the root element, then size children with the @-prefixed variants (@sm:, @md:, @lg: …).
• ❌ Never use viewport media queries (sm:/md:/lg:) or useBreakpoints/window width for a block's internal layout — those track the screen, not the pane. (Rejected in #710 for exactly this reason.)

<template>
  <!-- @container makes the @-variants below resolve against THIS element's width -->
  <div class="@container h-full overflow-auto">
    <div class="grid grid-cols-1 @md:grid-cols-2 gap-4">…</div>
  </div>
</template>

Reference implementations in packages/crouton-core/app/components/: LayoutSpikeList.vue, LayoutSpikeForm.vue, LayoutSpikeStats.vue (each uses @container; throwaway spike blocks, but the responsiveness pattern is the model).

Declare the sizing contract

On the block's CroutonLayoutBlockDefinition (in the package's app.config.ts croutonLayoutBlocks), declare what it needs. All optional; undeclared ⇒ fully fluid (minWidth 0).

Field	Use it for
minWidth	The viability floor (px). Below this the block breaks → the pane refuses to shrink past it, and the viability check flags it. Set it to the smallest width where the block is still legible after its narrowest container-query state kicks in.
minHeight	Smallest usable height (px).
maxWidth	Don't stretch past this (px).
defaultSize	Preferred size (% of its pane group) when first placed.
resize	'free' (default) · 'fixed' · 'aspect'.
aspect	width/height ratio when resize: 'aspect'.
density	'compact' | 'comfortable' | 'spacious' hint for the block's own layout.

Pick minWidth after you've authored the narrowest container state — it's the point past which even the collapsed layout stops working, not the desktop width.

List playbook

• Column priority + collapse order: decide which columns drop first as the container narrows; keep the identifying column (title) longest.
• Table → cards threshold: below a container width, switch a table to stacked cards (@container variant) — never a broken horizontal scrollbar.
• Min column widths so columns don't crush; sticky header on scroll.
• Empty / loading / error states all sized for the pane (no fixed-px blocks that overflow a narrow column).

Form playbook

• Two-column ⇄ single-column at a container threshold (@md:grid-cols-2 → grid-cols-1), not a viewport one.
• Label placement: top labels survive narrow panes better than left labels.
• Field grouping stays intact across the collapse; primary action stays reachable (sticky footer or end of flow), full-width when narrow.

General rules

• No fixed pixel widths that can overflow a pane — use intrinsic / fluid sizing.
• Contain overflow: overflow-auto on the scroll root; the block fills its pane (h-full w-full) and scrolls internally rather than pushing the pane.
• Honour your own minWidth — the declared floor and the narrowest CSS state should agree (don't declare 240 if the layout visibly breaks at 300).
• SSR-safe: no window/measurement on first render (container queries are pure CSS, so they work in SSR — that's another reason to prefer them).

Check your work

1. Drop the block into a wide pane, then drag it narrow — it should reflow (columns collapse / table→cards / 2col→1col) via container queries, not scroll sideways or crush.
2. The pane should refuse to shrink the block below its minWidth.
3. Run the viability metric over a layout that puts the block in a too-small pane — useCroutonLayoutBlocks().checkViability(tree, [1280, 768, 375]) (or the pure checkLayoutViability) should flag it. See crouton-core CLAUDE.md.