improvement(spacing): let Stack flip direction from CSS via stackBelow (CUI-38)

[JeanMarcMilletScality]

TL;DR — Stack gains stackBelow={px}: below that container width a horizontal Stack becomes vertical, and its separators switch to the vertical treatment along with it. Nothing renders differently today — the prop is opt-in.

Context / Why

Stack picks its separator element from the direction prop, at render time. A CSS flex-direction flip — which is how container-query-driven responsive layouts work — leaves the separators pointing the wrong way. A consuming application hits this the moment it needs a row of columns to collapse into a stack as its container narrows — the row cannot flip, so the content clips silently instead. CUI-38

🧩 Approach

This PR replaces an earlier revision that made the row separator self-orienting. That version used flex-basis for the main axis and align-self for the cross axis, so a CSS flip turned the rule into a full-width one. Design rejected it: a column Stack must keep its short dash. The two treatments are different shapes, not one shape on two axes — and CSS cannot query a parent's flex-direction — so the component has to emit the query itself.

Both treatments now live in one element as two css blocks, and Stack gains stackBelow:

const Separator = styled.div`
  background: ${(props) => props.theme.border};
  ${(props) => (props.$direction === 'horizontal' ? ruleSeparator : dashSeparator)}

  ${(props) => props.$stackBelow !== undefined && css`
    @container responsive (max-width: ${props.$stackBelow}px) {
      ${dashSeparator}                 // ←
    }
  `}
`;

One element is what makes this possible at all: CSS can restyle an element, but it cannot swap which element React rendered.

This follows a convention the library already has — a numeric breakpoint the component turns into an @container responsive query:

Component	Prop / constant	Below the width
Button	iconOnly={number}	label collapses to icon + tooltip
Form	STACK_BELOW	label/field flip to a stacked column
Stack	stackBelow={number} (new)	direction flips to vertical

Without an ancestor establishing that container (<Box container>), the query never matches and the Stack stays horizontal — the same graceful no-op as iconOnly.

Layout moved off Box's styled-system props onto a styled(Box) wrapper so the container query doesn't have to out-specify them. This is what removes the && specificity hack a consumer previously needed.

Verified no-op in both default directions

Rendered the old and new Stack side by side with ServerStyleSheet, then diffed the computed declarations per element (not the raw stylesheet, which differs only in class bookkeeping and rule splitting):

Default direction	Container	Separator
horizontal	identical (4 declarations)	identical (5 declarations)
vertical	identical (4 declarations)	align-self: auto and margin: 0 added

Those two additions are the CSS initial values for a flex item, so the computed style is unchanged. They exist only so the shared dashSeparator block resets what ruleSeparator sets when the container query swaps treatments.

The   text nodes are dropped from both separators. Each treatment sets explicit dimensions, so neither depends on content for its size.

Why the vertical dash had to stay — beyond design's ruling, a full-width rule in a page Form duplicates the header's own border-bottom: 1px solid theme.border (Form.component.tsx:102): same weight, same colour, near-same width, so a section separator becomes indistinguishable from the form title's rule.

📷 Screenshots

🔧 Usage

No existing Stack usage changes. What changes is how a consumer opts into a responsive direction — before, they hand-wrote the query and fought Box for specificity:

Before — what the earlier revision of this PR required:

const ResponsiveStack = styled(Stack)`
  @container (max-width: 500px) {
    && {                          // ← doubles specificity to beat Box's flex-direction
      flex-direction: column;
      align-items: stretch;       // ← must be overridden too, separately
    }
  }
`;

<ResponsiveStack withSeparators gap="r24">…</ResponsiveStack>

After — from the story added in this PR:

<Stack withSeparators gap="r24" stackBelow={500}>…</Stack>   // ←

Two notes for consumers:

• An ancestor must establish the responsive container — <Box container>. Without one the query never matches and the Stack stays horizontal.
• stackBelow is ignored when direction="vertical". Flipping a vertical Stack to a row in CSS is still unsupported — pre-existing behaviour, unchanged here.

🔍 Review focus

• 🟡 Moderate — src/lib/spacing.tsx › StackBox — layout (display, flex-direction, align-items) moved off Box's styled-system props onto a styled(Box) wrapper. This is the change most able to affect existing call sites: every Stack in every repo now gets its layout from a different class. The computed-declaration diff above is the evidence it's a no-op; gap still comes from Box, so the two classes now split what one used to emit.
• 🟡 Moderate — src/lib/spacing.tsx › Separator — ruleSeparator/dashSeparator must stay mutually resetting. dashSeparator restores align-self and margin precisely because ruleSeparator sets them; dropping either reset silently breaks the flipped state, and container queries don't evaluate in jsdom so no unit test will catch it.
• ⚪ Minor — src/lib/spacing.tsx › Stack — stackBelow is additive and Separator was never exported, so no public API is removed.

🧪 How to test

1. npm run storybook → Components/Styling/Spacing Utils › stackBelow — direction follows the container width.
2. Drag the container's bottom-right resize handle below 500px. The stack flips to a column and the separators become short dashes; drag back above 500px and they return to full-height rules. Nothing re-renders — only CSS changes.
3. Regression check on Stack Story: both the Banner example (horizontal) and the Vertical divided example must be pixel-identical to development/1.0.
4. Same for the vertical production sites under Templates/Form (page-form, tab-form, form-with-accordion) — section separators should be unchanged short dashes, still clearly subordinate to the form title's rule.

🚧 Follow-up

• Downstream pickup is a normal @scality/core-ui version bump once this is released; the integration details live in the consuming application's own ticket.
• The consumer layout that prompted design's ruling on the dash should be reviewed with them in its narrow state before the downstream change merges.
• A vertical Stack still can't be flipped to a row in CSS. No consumer needs it today. Not yet ticketed.
• Wrap is deliberately untouched — it does not wrap despite its name, deferred pending a usage audit. Not yet ticketed.

🔗 References

• CUI-38 — the ticket still describes the self-orienting approach, which design has since rejected. It needs updating to match this PR.
• CUI-39 / bugfix(icon): keep IconWrapper circular under flex pressure (CUI-39) #1183 — sibling fix from the same responsive work (IconWrapper deforming into an ellipse), merged; released in 0.228.0.

What changed

src/lib/spacing.tsx — HSeparator/VSeparator/Separator({ type }) collapse into one Separator styled component carrying both treatments as ruleSeparator and dashSeparator css blocks. A new StackBox = styled(Box) holds the layout so the container query doesn't fight styled-system for specificity. Stack gains stackBelow?: number and passes it, with direction, to both as transient props (verified not to reach the DOM).

stories/spacing.stories.tsx — replaces the earlier ContainerQueryDirectionFlip story with StackBelowStory, which uses the prop instead of a hand-written query. Its container sets container-name: responsive explicitly, documenting the requirement.

Deliberately not in this PR: Wrap (out of scope per CUI-38), any restyle of existing vertical separators, and any Form change.

tsc --noEmit clean, npm run build clean, npm run lint clean, suite green.

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