feat(react-icons): expose the bundled icon variant as a data attribute

[Ray Knight (ArrayKnight)]

bundleIcon renders both glyphs of a pair and hides one with data-fui-icon-hidden. A consumer
stylesheet that wants to style the two variants differently — or select the visible one — currently
has to key off the fui-Icon-filled / fui-Icon-regular class names, which are a rendering detail
rather than a contract.

This adds data-fui-icon-variant="filled" | "regular" to the glyphs bundleIcon renders, exported
as DATA_FUI_ICON_VARIANT alongside the existing DATA_FUI_ICON, DATA_FUI_ICON_RTL,
DATA_FUI_ICON_HIDDEN and DATA_FUI_ICON_FONT. It is additive: no existing attribute, class name
or export changes, and nothing renders differently.

The headless entrypoint's bundled-variant test coverage is extended to assert the attribute on both
glyphs in both filled states, and both copies of the attribute documentation
(docs/headless.md and the docsite mirror) are updated.

Context. This comes out of building a complete Tailwind v4 + CSS Modules styling layer over
@fluentui/react-headless-components-preview in microsoft/fluentui — see microsoft/fluentui#36656. That
layer styles icons entirely from CSS, and the class-name route was the one place it had to depend on
a string rather than a contract. It is useful independently of that work: any consumer styling
headless icons hits the same wall.

fix(react-icons): dedupe CJS rewrite targets so concurrent rewrites cannot tear a file

(separate commit, unrelated to the above — happy to split into its own PR if preferred)

finalizeCjs collected rewrite targets into an array. A stale pre-renamed file and its fresh source
can resolve to the same target path, so the same file was rewritten twice concurrently and torn.
Collecting into a Set makes the target list unique.

docs(react-icons): explain how the hidden-variant rule is meant to be layered

(comment-only commit; drop it if you would rather not carry it)

[data-fui-icon-hidden] is the one rule in headless/styles.css consumers routinely override — any
hover or selection glyph swap reveals the inactive variant. Every other non-obvious rule in that
file carries a rationale block; this one did not, so the constraint it places on a consumer (import
the stylesheet below your component layers, then a normal declaration wins) was only discoverable by
experiment. The declaration itself is untouched — it matches upstream/main and published npm
(2.0.339) byte for byte.

---

Why a data-* attribute rather than the class names

Selecting on fui-Icon-filled couples a consumer's stylesheet to a string the package is free to
change. The DATA_FUI_ICON_* family is the surface the headless entrypoint already treats as its
public selector contract — data-fui-icon, data-fui-icon-rtl, data-fui-icon-hidden,
data-fui-icon-font. The variant is the one gap in that family, and it is the fact a CSS-first
consumer reaches for most, because a bundleIcon pair is precisely the case where "which glyph is
this?" has to be answerable from CSS alone.

On the name. The attribute is namespaced to match every sibling constant:
data-fui-icon-variant / DATA_FUI_ICON_VARIANT. A bare data-variant on every bundled glyph
would collide with consumer and design-system attributes, so the short spelling was ruled out before
this PR rather than argued for in it.

What changed, file by file

feat commit — packages/react-icons/src/headless/{shared.ts,index.ts} and bundleIcon. Adds
export const DATA_FUI_ICON_VARIANT = 'data-fui-icon-variant', re-exports it, and spreads
{ [DATA_FUI_ICON_VARIANT]: 'filled' } / 'regular' onto the two glyphs bundleIcon renders.

Test. headless.test.tsx gains
bundleIcon stamps data-fui-icon-variant on both glyphs regardless of filled, asserting both glyphs
in both filled states. The pre-existing "bundleIcon renders both variants with data attributes"
test (which asserts only DATA_FUI_ICON_HIDDEN) is left as-is. Suite: 24 passed, was 23.

Docs. Both copies updated — packages/react-icons/docs/headless.md and its mirror
packages/docsite/stories/Icons/Headless/IconsHeadless.md. Each gains the constant in the export
list and a bundleIcon variant row in the "How it works" table naming the fui-Icon-filled /
fui-Icon-regular class names as the Griffel-side equivalent. Prettier reflowed the table's second
column as a result; that column-width churn is the only incidental diff in either file.

One caveat for the reviewer, pre-existing and left alone. The docsite mirror tells readers to
import these constants from @fluentui/react-icons/headless/utils, but src/headless/utils.ts
re-exports only the class-name constants, never the DATA_FUI_ICON_* ones — they come from the
./headless barrel. That inaccuracy predates this change; worth fixing separately.

fix commit — packages/react-icons/scripts/module-format.js, in finalizeCjs: toRewrite
changes from a string[] with .push() to a Set<string> with .add(), spread at the
forEachConcurrent call site. Genuinely independent of everything else here — it has nothing to do
with Tailwind, styling, or the attribute above.

Relationship to the companion microsoft/fluentui PR

The dependency is real and narrow. The styling layer imports @fluentui/react-icons/headless in
106 files, but that entrypoint is not what it needs this PR for — the whole src/headless/
tree and its ./headless* export conditions already exist here.

What it needs is the data-fui-icon-variant stamp specifically. It publishes two @custom-variant
definitions against the attribute in a documented public export of its own package, and every
component that swaps a glyph — Button, ToggleButton, MenuButton, MenuItem,
BreadcrumbButton, Tab — reaches the attribute only through those two variants. Without this
change, those rules match nothing, silently.

So the linkage is: this PR opens first; the companion PR keeps a pinned resolution until this one
merges and publishes, then moves to the published version and drops it. Cross-links in both
directions.

Changelog. No change file: this repo uses neither beachball nor changesets. It runs
@commitlint/config-conventional with nx release driven by conventional commits, and
packages/react-icons/CHANGELOG.md is machine-generated with PR backlinks. All three commit
subjects are valid conventional commits.