Add fluid spacing support to theme.json (parity with fluid typography) #81864

What problem does this address?

Since 6.1 (#39529), fluid typography in theme.json is first-class: settings.typography.fluid, per-preset fontSizes[].fluid: { min, max }, and auto-generated clamp() for --wp--preset--font-size--*.

Spacing has no equivalent. spacingSizes[].size only accepts a raw CSS string so fluid spacing means hand-written clamp() in every preset (handbook).

Spacing also differs from typography in that it already has settings.spacing.spacingScale to auto-generate a scale from a few parameters. That scale only produces static values, though. Fluid spacing forces authors to abandon spacingScale and hand-author every entry in spacingSizes.

Missing compared to typography:

• Declarative min / max per preset
• Global viewport settings (minViewportWidth / maxViewportWidth)
• Per-preset opt-out via fluid: false
• A consistent model for the upcoming spacing presets UI (#61986, #72301)

This was discussed early on (#39371) but never implemented in the theme.json engine.

What is your proposed solution?

Mirror fluid typography for spacing presets:

{
  "settings": {
    "spacing": {
      "fluid": {
        "minViewportWidth": "768px",
        "maxViewportWidth": "1280px"
      },
      "spacingSizes": [
        {
          "name": "Medium",
          "slug": "md",
          "size": "1.75rem",
          "fluid": { "min": "1.5rem", "max": "1.75rem" }
        },
        {
          "name": "Small",
          "slug": "sm",
          "size": "1rem",
          "fluid": false
        }
      ]
    }
  }
}

Expected output:

--wp--preset--spacing--md: clamp(1.5rem, …, 1.75rem);
--wp--preset--spacing--sm: 1rem;

As a follow-up, fluid support could also extend to spacingScale. Today it generates a static scale from operator, increment, steps, mediumStep, and unit (with mediumStep always mapped to slug 50). When spacing.fluid is enabled, the same generator could emit fluid steps from a min/max medium value, for example:

{
  "settings": {
    "spacing": {
      "fluid": {
        "minViewportWidth": "768px",
        "maxViewportWidth": "1280px"
      },
      "spacingScale": {
        "operator": "*",
        "increment": 1.5,
        "steps": 7,
        "unit": "rem",
        "fluid": {
          "min": { "mediumStep": 1.5 },
          "max": { "mediumStep": 1.75 }
        }
      }
    }
  }
}

That would generate both ends of the scale from the same rules, then clamp each step (e.g. --wp--preset--spacing--50: clamp(1.5rem, …, 1.75rem)), instead of requiring a fully manual spacingSizes list.

Implementation: reuse Core math (wp_get_typography_font_size_value(), wp_get_computed_fluid_typography_value()) during WP_Theme_JSON preset resolution. Longer term, extract a generic wp_get_fluid_size_value() shared by typography and spacing.

Workarounds today

1. Manual clamp() in size but verbose and hard to maintain
2. Theme polyfill via wp_theme_json_data_theme, works but shouldn't be necessary. Declaring settings.spacing.fluid and spacingSizes[].fluid also invalidates the official theme.json schema, since those properties are not part of it today.

Related: #39371 · #61986 · #72301 · #39529 · #41527