fix(IssueLabelToken): Fix lightness-switch formula for label text contrast

[Copilot]

Ports the --lightness-switch formula fix from github/github-ui#13332 to IssueLabelToken.

The multiplication-based formula (perceived-lightness - threshold) * -1000 does not reliably produce a binary 0/1 switch for all color inputs — certain label colors near the threshold get intermediate values, causing incorrect text contrast in light theme. The division-based formula 1 / (threshold - perceived-lightness) produces ±infinity, which max(0, min(..., 1)) correctly clamps to a clean 0 or 1.

/* Before */
--lightness-switch: max(0, min(calc((var(--perceived-lightness) - var(--lightness-threshold)) * -1000), 1));

/* After */
--lightness-switch: max(0, min(calc(1 / (var(--lightness-threshold) - var(--perceived-lightness))), 1));

Changelog

New

N/A

Changed

• IssueLabelToken: Updated --lightness-switch CSS formula to use division-based approach for correct black/white text selection on colored labels

Removed

N/A

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Testing & Reviewing

Compare IssueLabelToken rendering with light-colored fill values (e.g. #e8e356, #bfd4f2) in light theme — text should be dark. This is a pure CSS change with no API surface change.

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github-ui (Learn more about how to run integration tests)

[changeset-bot]

🦋 Changeset detected

Latest commit: 8d5836d

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/react	Patch

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Click here if you're a maintainer who wants to add another changeset to this PR

[github-actions]

⚠️ Action required

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. Check the integration testing docs for step-by-step instructions. Or, apply the integration-tests: skipped manually label to skip these checks.

To publish a canary release for integration testing, apply the Canary Release label to this PR.

[francinelucca]

[Copilot]

Pull request overview

This PR updates the CSS logic used by IssueLabelToken to compute a binary light/dark text switch based on perceived lightness, fixing incorrect contrast selection for some label colors in light theme.

Changes:

• Replace the multiplication-based --lightness-switch formula with a division-based formula that clamps cleanly to 0/1.
• Add a patch changeset for @primer/react documenting the fix.

Show a summary per file

File	Description
packages/react/src/Token/IssueLabelToken.module.css	Updates --lightness-switch calculation to reliably clamp to a binary value for text color selection.
.changeset/fix-issue-label-token-contrast.md	Adds a patch changeset entry describing the contrast fix for IssueLabelToken.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 0